AVG7 is good with regular updates and like Avast, it is free. Avast is actually better than AVG in terms of detection, but personally, i prefer AVG. Nod is much better than both, but is a paid for application.
The key to security is to have a layered defence, for example
Firewall - Zonealarm Free
AV - AVG7
AT - ewido (loses real time protection after trial expires, but it still works otherwise)
AS - Spysweeper (free trial, then u goota buy it, but its good)
in addition, i always install
Adaware SE Free
Not much gets past that lot.