help, trojans wont go away

Hi there, my pc keeps crashing and resetting, mainly freezing. Im using avast and ewido and it keeps finding the same 13 trojans (downloader.agent.uj). it seems to deal with them but then the next time i start up its back to square one, do a scan and there they are again. any ideas? ive tried doing it in safe mode etc. i am very new at this so bear with me!! any suggestions greatly appreciated, cheers pete