Besides what David said, you can schedule a boot time scanning with avast. Start avast! > Right click the skin > Schedule a boot-time scanning. Select for scanning archives. Boot. Other option is scanning in
SafeMode (repeatedly press F8 while booting).
If you still detecting any strange behavior or even you're sure you're not clean, maybe it will be good to test your machine with
anti-rootkit applications. I suggest
AVG or
Panda.
Also, if you still detecting strange behaviors or you want to be sure you're clean, maybe making a
HijackThis log to post here and, specially, scan and submit to on-line analysis the
RunScanner log would help to identify the problem and the solution.