Are you using Windows XP?

If so, disable (and enable it after) System Restore

Start > Control Panel > System > System restore > Disable
Click Apply
Enable it again
Click Ok

It will be good too if you delete the temporary Internet files?
To do this go to Internet explorer >Tools > Internet options > Delete files > Click delete all offline content (just to be sure) > click ok.
It might take some time to delete them.

After that, schedule a boot-time scanning:
Start avast! > Right click the skin > Schedule a boot-time scanning
Select for scanning archives.
Boot.