There is a similar topic http://forum.avast.com/index.php?topic=25822.0
Use this tool to scan your PC - http://www.f-secure.com/blacklight/
Or any other antirootkit like Panda, AVG or BitDefender.

You probably still have an infection, probably a rootkit on your system that is killing avast.

See also:
http://forum.avast.com/index.php?topic=26554.0
http://forum.avast.com/index.php?topic=25941.0
http://research.pandasoftware.com/blogs/research/archive/2006/12/14/Rootkit-cleaner.aspx
http://www.f-secure.com/blacklight/try_blacklight.html

After running the above rootkit tools if nothing is found try these.
If you haven’t already got this software (freeware), download, install, update and run it, preferably in safe mode.
AVG anti-spyware or SUPERantispyware or Spyware Terminator.