I have limited resources (read RAM) so I tend to pay a lot of attention to this sorta thing, and AVG worked really well for me, until my fully updated version recently detected a trojan but couldnt block it, move it to the quarantine vault much less remove it. I then went to BitDefender's online scanning engine and it detected (and successfully removed) a few infected files. :-o My 2 cts on this matter: invest in a personal firewall at least (or get Zone Alarm, there's a free download for personal use), practice vigilance when dealing with strange files and get a good antivirus, most do the job of detection, blocking and removal of spyware as well. If you can afford the cost and the processing power, an anti-spyware software should provide an extra layer of protection for your computer as that's their specialty. The good ones tend to hog memory and processing power (though I doubt you will notice it on your machine) but it's worth it, considering your precious system and all the information in it is at risk.
Take a look at these reviews on anviruses:
PCWorld review (March 2006 issue)
http://www.pcworld.com/reviews/artic...,124163,00.asp
Top Ten Reviews
http://anti-virus-software-review.toptenreviews.com/
Anti-spyware reviews:
PCWorld review (April 2006 issue)
http://www.pcworld.com/reviews/artic...,119572,00.asp
Top Ten Reviews
http://anti-spyware-review.toptenreviews.com/
There are infinitely more reviews out there but I feel these really helped me, and besides being very recently written, they're both comprehensive and easy to understand as well.
Do try a few demo versions before deciding on the one you like best ie in terms of mem usage, interface, features etc.
Last but not least, remember to update your virus .dat files religiously!
edit: minor adjustments to text, just remembered about ZA free
