Depends on 32 or 64 bit for antivirus paid or free

Concerning whether anti-virus is better when paid for or not -it depends, on your budget, and compatibility issues. For example if you are using 64 bit instead of 32 bit you may be limited as to what you can use.

32 bit vs 64 bit
64 bit has been known to have issues with being limited to certain brands of anti-virus software. "Avast" antivirus or "C/A" (Computer Associates) is something that I know works. If you have 32 bit XP Pro or Vista 32 Pro instead of 64 bit it opens some more doors for you. The new Vista 64 bit non-Beta comes out in Feburary and once the bugs are worked out the software limiting factor may be corrected.