I am posting in part to help get this back to the top so that others can see it again.
As I understand it, RAM is only in use whilst the PC is active and when you turn the PC off the RAM is cleared. IF you have an infection then it is most likely to be in the boot sector or the harddrive somewhere.
You say you have stopped everything loading at startup - but how? If you simply disabled from the startup folder in windows, then I would suggest using MSConfig [start>run> type MSCONFIG and click on run]. Have you tried starting in safe mode and then loading each driver in turn as you may be able to verify a rogue/corrupted file? 3 virus scans would tell me that you are unlikely to be infected - subject to them all being up to date of course - but bringing this back to the top might get another posting from someone better qualified than I.