virus help

How can I determine if I have a virus if I can never get a virus scanner to run completely? I have tried numerous software packages and it does not seem to fix the problem. I am almost sure that this is virus related and don;t want to format my machine. (I Just did it for maintenance purposes earlier this month.)The programs will freeze randomly in diffently locations. Otherwise I have no problems with the system freezing and surf the net and do other things without incident. I have WIndows 98SE with all Windows critical updates being done.