Avast 4.8 should work just fine, though I never really cared much for the user interface. Version 5 has a much nicer looking user interface and adds a new feature called Virtualization (sandbox).
You might also want to consider Avira. I use Avira AntiVir Premium (v.9) on my Windows 7 64 bit desktop. Works great and is very light on system resources. I'm also beta testing Avira AntiVir Premium Security Suite 10, which adds thier new ProActiv behavior-based detection technology.