I'll toss in my vote for virus as well. This is a well known tactic of virii. Disables your ability to open regedit, msconfig, and taskmanager, as well as usually attempting to disable AV software.
My advice is to boot into safe mode as suggested earlier by Poertner, look for any strange looking services, try to disable them from startup, or, after locating the potential culprit service(s) post back and someone can probably help you with specific instructions on removing the particular virus.
**Just because your AV software doesn't find anything doesn't mean the system is clean, this could be something that has come along since you last updated.