Virus/Trojan

[rajpoot]

So someone did certain things on my PC that I never do, and as a result when I switched on my PC for a relaxing hour after work today, I realised it was infected with a virus...or trojan....I can never remember how they differ.
Anyway, let me begin with the specifications. I'm still running Windows XP SP2, and it's been working perfectly fine up till now. Furthermore I didn't have an Antivirus software installed since my previous experiences with McAfee, Norton and Kapersky haven't been good. More often than not they ended up deleting my compiled executables rather than detect and delete actual viruses.
Now each of my drives have an autorun.inf in the root, with a .pif file and a .exe file. I can manually delete the exe files and pifs, but the autorun.inf all seem to be used by some process so I cannot delete them. Furthermore deleting the other files has been ineffective up till now since they are replaced quickly.

I finally caved in and installed AVG, this evening, but till now it has done nothing useful and has simply been eating up my processor and RAM.
I also tried to kill the process which was using the autorun.inf files, but I couldn't find it, and the unlocker utility isn't working.

So basically I'm in a bad fix. The PC is a big part of my everyday life, and worst of all, the unwanted processes running hidden seem to be hooking SecuROM software so quite a few games installed have stopped working.

Any sort of help and/or suggestions would be appreciated, because I really don't want to go through the hassle of taking backup of my drives and then formatting them and reinstalling Windows.

Thank you, for any help anyone can provide.

Edit:
Now AVG seems to have done something to my programs since Windows can no longer find the path to the executable files like Free Commander, even though they're still where they were.