Thats because no antivirus solution can detect everything, it's just impossible. The malware writers are always one step ahead of AV companies because they have to discover them. Avast's detection is around 97% from what I know.
My suggestions:
- Pair up your antivirus product with
Malwarebytes and
Hitman Pro. They work great alongside
antivirus programs and they're free.
- If you have avast Pro or Internet Security, use the
Process Virtualization module to prevent any future infections.
Or use
Sandboxie if you want a free version (a bit harder to use imo).