Never had a problem with malware and Avast, and I visit questionable sites. Have you set up an automatic scanning schedule and what to do with the files found? Avast is great I love the sandboxxing feature.
It sounds like you have another infection that is allowing the malware entry. If you have some comfort using computers, you can try downloading some software called HIREN's BOOT DISK this allows you to remove infections (and all kinds of other neat things) without loading ANYTHING from your PC, and it has virus scanners built in.
Another suggestion would be to run "microsoft safety scanner" from microsoft.com.
PS Oh and make sure to run FULL scans not quickies!