What if my computer is infected?

Unfortunately, it may happen occasionally that the antivirus installed in your computer with its latest updates is incapable of detecting a new virus, worm or a Trojan. Sadly but true: no antivirus protection software gives you a 100% guarantee of complete security. If your computer does get infected, you need to determine the fact of infection, identify the infected file and send it to the vendor whose product missed the malicious program and failed to protect your computer.

However, users on their own are typically unable to detect that their computer got infected unless aided by antivirus solutions. Many worms and Trojans typically do not reveal their presence in any way. By way of exception, some Trojans do inform the user directly that their computer has been infected – they may encrypt the user’s personal files so as to demand a ransom for the decryption utility. However, a Trojan typically installs itself secretly in the system, often employs special disguising methods and also covertly does its activity. So, the fact of infection can be detected by indirect evidence only.

Symptoms of infection

An increase in the outgoing web traffic is the general indication of an infection; this applies to both individual computers and corporate networks. If no users are working in the Internet in a specific time period (e.g. at night), but the web traffic continues, this could mean that somebody or someone else is active on the system, and most probably that is a malicious activity. In a firewall is configured in the system, attempts by unknown applications to establish Internet connections may be indicative of an infection. Numerous advertisement windows popping up while visiting web-sites may signal that an adware in present in the system. If a computer freezes or crashes frequently, this may be also related to a malware activity. Such malfunctions are more often accounted for by hardware or software malfunctions rather than a virus activity. However, if similar symptoms simultaneously occur on multiple or numerous computers on the network, accompanied by a dramatic increase in the internal traffic, this is very likely caused by a network worm or a backdoor Trojan spreading across the network.

An infection may be also indirectly evidenced by non-computer related symptoms, such as bills for telephone calls that nobody made or SMS messages that nobody sent. Such facts may indicate that a phone Trojan is active in the computer or the cell phone. If unauthorized access has been gained to your personal bank account or your credit card has bee used without your authorization, this may signal that a spyware has intruded into your system.

What to do

The first thing to do is make sure that the antivirus database is up-to-date and scan your computer. If this does not help, antivirus solutions from other vendors may do the job. Many manufacturers of anti-virus solutions offer free versions of their products for trial or one-time scanning – we recommend you to run one of these products on your machine. If it detects a virus or a Trojan, make sure you send a copy of the infected file to the manufacturer of the antivirus solution that failed to detect it. This will help this vendor faster develop protection against this threat and protect other users running this antivirus from getting infected.

If an alternative antivirus does not detect any malware, it is recommended that you disconnect your computer from the Internet or a local network, disable Wi-Fi connection and the modem, if any, before you start looking for the infected file(s). Do not use the network unless critically needed. Do not use web payment systems or internet banking services under any circumstances. Avoid referring to any personal or confidential data; do not use any web-based services that require your screen name and password.

How do I find an infected file?

Detecting a virus or Trojan in your computer in some cases may be a complex problem requiring a technical qualification; however, in other cases that may be a pretty straightforward task – this all depends on the degree of the malware complexity and the methods used to hide the malicious code embedded into the system. In the difficult cases when special methods (e.g. rootkit technologies) are employed to disguise and conceal the malicious code in the system, a non-professional may be unable to track down the infected file. This problem may require special utilities or actions, like connecting the hard disk to another computer or booting the system from a CD. However, if a regular worm or simple Trojan is around, you may be able to track it down using fairly simple methods.

The vast majority of worms and Trojan need to take control when the system starts. There are two basic ways for that:

  • A link to the infected file is written to the autorun keys of the Windows registry;
  • The infected file is copied to an autorun folder in Windows.

The most common autorun folders in Windows 2000 and XP are as follows:

%Documents and Settings%\%user name%\Start Menu\Programs\Startup\
%Documents and Settings%\All Users\Start Menu\Programs\Startup\

There are quite a number of autorun keys in the system register, the most popular keys include Run, RunService, RunOnce и RunServiceOnce, located in the following register folders:


Most probably, a search at the above locations will yield several keys with names that don’t reveal much information, and paths to the executable files. Special attention should be paid to the files located in the Windows system catalog or root directory. Remember names of these files, you will need them in the further analysis.

Writing to the following key is also common:


The default value of this key is “%1″ %*”.

Windows’ system (and system 32) catalog and root directory are the most convenient place to set worms and Trojans. This is due to 2 facts: the contents of these catalogs are not shown in the Explorer by default, and these catalogs host a great number of different system files, functions of which are completely unknown to a lay user. Even an experienced user will probably find it difficult to tell if a file called winkrnl386.exe is part of the operating system or foreign to it.

It is recommended to use any file manager that can sort file by creation/modification date, and sort the files located within the above catalogs. This will display all recently created and modified files at the top of the catalog – these very files will be of interest to the researcher. If any of these files are identical to those occurring in the autorun keys, this is the first wake-up call.

Advanced users can also check the open network ports using netstat, the standard utility. It is recommended to set up a firewall and scan the processes engaged in network activities. It is also recommended to check the list of active processes using dedicated utilities with advanced functionalities rather than the standard Windows utilities – many Trojans successfully avoid being detected by standard Windows utilities.

However, no universal advice can be given for all occasions. Advanced worms and Trojans occur every now then that are quite difficult to track down. In this case, it is best to consult the support service of the IT security vendor that released your antivirus client, a company offering IT assistance services, or ask for help at specialized web forums. Such web resources include www.virusinfo.info and anti-malware.ru (Russian language), and www.rootkit.com and www.gmer.net (English). Similar forums designed to assist users are also run by many antivirus companies.

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