As I had problems with tasks on my PC, too, here are some questions:
- Can you see any tasks in Control Panel > Scheduled Tasks?
- If not, do all the tasks disappear for good, or do they re-appear after restart?
- Do you have access to the %SystemRoot%\Tasks folder, or the system refuses access?
- What does display the command "schtasks /query", run from the command line?
- And finally, don't you have Bitdefender Free Edition installed? — That was the culprit in my case.
(I'm using WinXP, so some locations or names might be different.)