I've used AVG, Antivir, Norton, Mcafee, Panda, Pccillin.... over the years. IMO, all are excellent, and the free ones work just as well.
I have no warm feelings for Norton/Mcafee. They tend to be resource hogs.
Any antivirus program is only as good as the user. Update it regularly and perform regular scans.
Also, the default settings seem to be lacking with all of them. Enable the more detailed features.
Housecall still has their free online scanner. This way if you grab one of the free AV programs, you can still run a competing product once every now and then to see if anything questionable has been missed.
You can supplement your free slim AV with regular Spybot and Adaware scans, which are also free to scan, clean, and update, but not run in the background. Spywareblaster and Spybot have immunizations that can help. AVG, Tenebril, also have free antispyware programs too.
Spysweeper, Counterspy, Pestpatrol, SpywareDoctor... are also pretty good but cost to use. There is another program, free to use, but can't remember its name. I'll have to check my PC when I get home.
For a firewall, XP has its own that is good enough. Zonealarm is pretty good and free for older MS operating systems. A router makes firewall choice less of an issue.
Keep your software as up to date as possible.
I have no issues with any of the programs. None have ever really failed me. I find that user error, lack of regular updates and scans, lack of software patches, settings not set well, and questionable surfing/downloading habits, can cause issues with ANY computer regardless of protection being used. .