Are you using 98 or ME as they were always associated with those kind of problems? If so, it's time to upgrade.
Also, AV's don't always find the culprits. It may make sense to look for it yourself. Take a look in the task manager & msconfig to see what's going on.
Can you remember the things you did (e.g download/install, CHAT, update, new hardware, etc) leading up to when it began? Try uninstalling stuff a bit at a time.
I used to get that problem with spybot sd. I think I set the security too high. So I uninstalled it but when a newer version came out I loaded it back on and now I have no more problems.