Anti-Virus software is one thing - but it can not protect 100%. No AV provides that. In fact, Avast is amongst the best concerning detection.
However, malware such as trojans, worms etc. have many ways to infiltrate. Opening unknown Emails, executing attachments...)
A good protection:
Avast5 (free)
Comodo firewall (free, be sure just to install the firewall)
ad-aware (free)
+
"Brain.exe" (no offense intended, but many people just think that with AV programs they're 100% safe - which is wrong)
Another good idea is to create a restricted user account for everyday purpose.
As to malware removal: this forum has lots of savvy guys that will take you step by step through the process if needed, with essexboy being the "man for the tough stuff".
Wishing you a virus free future
Greets
Zyndstoff