Anti-virus keeps popping up with the same message every few minutes, what is wrong with my computer?

Does this only occur when you're browsing (possibly shady) websites? If so, it sounds like Avast! is doing its job. If it happens even when you're not browsing, then there probably is a real problem. Try as many scanners as you can; you seem to be off to a good start. You can also try using System Restore to go back to a known good system state.