To begin with there are few details to work with. Afterall, we're not here to guess on even the basics of your PC. Don't know if this is a desktop or laptop, it matters.
Since, it so happens a flashdrive is involved, then check it by itself. It does no good to walk the flashdrive around if in fact its the source or malware or baddie. I suggest you get UBCD or Hiren's help discs. Burn both or either as a bootable disc and boot from it. It loads its own OS and allows some virus scanning once you direct it. Hiren's is more windoze related look and feel so it may seem familiar to you. You isolate any problem as you boot from separate source and can test. If it so happens the problem returns, then more likely you have power issues not some malware. I haven't see a virus attacking in such a matter before. If a autorun virus, you could maybe boot into safemode and check the msconfig status, look at "startup" files and see what's what. I also don't see what AV pgm. you're using but if it can boot by itself or check that it can, it could start upon boot-up and check status prior to any windoze loading. Do whatever correction and before rebooting see if you can scan the flashdrive as well. In fact, if you feel the flashdrive is the source of problem, leave it OUT entirely for days or weeks, get another one. if the problem doesn't return and then you mount problem flashdrive, then you know the real cause and zero-in on it.