Which AV software?

Norton Antivirus and ZoneAlarm Free version have always worked for me. Some vouch by the XP firewall. Probably there are as many opinions as products.

I'm always tempted to answer, "anything but McAfee" to this question however, I will just say NAV and NOD32 would be a the top of my "A-List".

I have used Mcafee for many years and have had very good luck with it. Many of the free ones lack some thing, some do a poor job on E-Mail, the major source of virus's.  No matter which you get update the Dat files weekly, set up the program to scan in the background (if it has it), scan all e-mail & web downloads, running it once in a while is a disaster! You might download Ad Ware 6 from lavasoft.com, clears up tracking bugs especially if you surf alot. From Mcafee their stinger.exe does a good job on trojans,worms and other really bad stuff only,it is NOT a full scan app. All of these programs, as any others must be updated , there are too many good hackers especially in the Asian areas. 

Good Luck and have fun computing!

Hooteye has good advice!