recently i faced two computer viruses - one is virut on my PC, and 2nd one is sality on my friend's pc.
both of them were detected by avast! AV scanner, as most of us using avast home/pro version for basic computing. but the fact is avast is not the best AV , rather it's best AV as freeware.
win32.virut or win32.sality have some common features - corrupt the .exe files. Deleting these infected files by ur antivirus means ur system gets unstable and u r forced to format ur XP. but as th contaminated most of the '.exe's on ur machine , if u store setups of various programs and run them to install programs after u format & reinstall XP . the infection comes back !! sometimes infection returns even from System Volume Information or Recycled folder - which is a hidden folder on each partition. and avast fails. even one of my fav virus cleaner combofix did nt work out !
virus does increase the size of the .exe files - just like laying eggs by houseflies ! and avast detects it as virus.
there are viruses hidden mostly in the folder of C:/Windows or C:/windows/system32 or C:/windows/system32/drivers
try to kill it via Safemode or using Window Washer(ww), run WW for 'free space washing' for deleting virus contaminated files in System Volume Information or Recycled folder. u should take a look what is running in background - use Process explorer.
the best antivirus to get rid of such infection on ur PC, is KASPERSKY ! it can heal the files ! this is the best part, which avast can't offer ! well KAV is not free, but u can try it for 30 days and to activate trial u need active internet connection or go for piracy - get new licenses as they appear on torrent sites.
*my post was made in hurry, but the conclusion is here : use kaspersky antivirus, as it works best on a moderate resourced PC ! best part is it will heal all files and make u back to work !
UPDATE One of my friends runs a cyber cafe, and as usual his WinXP-powered computers hit by a virus attack, though I am not sure that was virus or spyware. But that disabled Kaspersky Antivirus. I also checked on Task Manager some unwanted .exe files are running from C:/Documents and Settings/User/App Data/Temp [this may be wrong as I am currently on Win7]. I told him to go format way.
But for home computers, you can check out this helpline- BleepingComputer, it offers ComboFix.exe which saves my ass many times while I was WinXP.