Besides David suggestions, why don’t you:
- Disable System Restore and reenable it after step 3.
- Clean your temporary files.
- Schedule a boot time scanning with avast with archive scanning turned on.
- Test your machine with anti-rootkit applications. I suggest AVG or Trend Micro RootkitBuster.
But, specially:
5. Make a HijackThis log to post here or, better, submit the RunScanner log to to on-line analysis.