Actually (
) certain Trojan Horses contain code to stop a large quantity of AVs from running. It sits as a TSR that is run in your registry every time you boot, and kills the process before it gets chance to start.
The only TRUE way to disinfect your PC of viruses is to boot from a DOS disk, the downside of this is that most DOS disks don't recognise NTFS partitions, which you use mostly for Windows XP installs.
My advice is this:
Run "regedit" and back up!!!!!!!!!!
Go to: HKey_Local_Machine>Software>Microsoft>Windows>Current Version>Run
There should be a load of entries in there. These are the applications that run on startup, usually more important programs like driver applications etc.
Check through the files listed. If there is a name you are not sure of check the file's location. If it's in C:\Windows or C:\Windows32\System32 then beware. Any real system entries don't have "C:\windows\etc." in them. This is due to the system folders being part of your PATH entry, and they are located automatically.
If you have any suspect files in there, get an antivirus! if it won't open then you are infected and you'll have to take steps to disinfect.
This is the same for that cr*p you usually download when browsing (or letting your missis/kids use the PC!) like toolbars/search bars etc. These entries can usually be deleted without causing any problems.
That's about it! Keep your AV updated, don't download cr*p, use a decent firewall, and in the words of the good book... Don't Panick!