A file infecting virus, for example Virut, is able to modify itself each and every time it runs. In addition, when it infects, sometimes it will destroy the file it tries to latch onto. For these reasons, you really can't truly fix such infections. You will need to format/reinstall the operating system on this machine.
There are bugs in the viral code. When the virus produces infected files, it also creates non-functional files that also contain the virus.
W32/Virut.h is a polymorphic, entry point obscuring (EPO) file infector with IRC bot functionality. It can accept commands to download other malware on the compromised machine.
It appends to the end of the last section of executable (PE) files an encrypted copy of its code. The decryptor is polymorphic and can be located either:
Immediately before the encrypted code at the end of the last section
At the end of the code section of the infected host in 'slack-space' (assuming there is any)
At the original entry point of the host (overwriting the original host code)
Miekiemoes, an expert for malware removal, and an MS-MVP, additionally has a blog post about Virut.
I suggest that you backup all of your valuable data/documents/pictures/movies/songs/etc..
Do NOT backup any applications/installers and Do NOT backup any .exe/.scr/.htm/.html/.xml/.zip/.rar files...
This because these files may be infected as well. If you back them up and replace them afterwards, it will infect your computer again.
Read here for instructions how to format and reinstall Windows:
After you reinstall XP, it is extremely important that you install XP SP3. SP2 is no longer supported so you do not receive security updates from Microsoft. Those security updates are necessary to ensure that your computer will not be reinfected.
For more information on keeping your computer safe, please read Computer Security - a short guide to staying safer online.
I'd be grateful if you could reply to this post so that I know you have read it and, if you've no other questions, the thread can be closed.