Free ones can be just as good and some times better then paid ones.
Here is the ones i personally use and recommend to many of my customers.
Free versions:
Avast Free antivirus is by far the best out there when it comes to free anti virus programs, i have used it for many yrs and its been just perfect. (it has a gaming mode )
Avira is another good one if you dont like Avast, i find it works better on a windows XP machine over Avast, ive seen avast screw up a few computers once it was updated to Version 7.
Comodo Free Firewall is the best firewall i have used and i have used it for a good 7yrs, its been great, ive tried many other freebees but in the end i still come back to Comodo, works good on any system, XP/Vista/7. The anti virus that comodo also has is good but i have found it to stuff up the odd Vista based system, but over all its pretty good. (also has a gaming mode)
Payed versions:
Kaspersky is the best i think, it can be a bit heavy and the updates are as slow as fuck, but when it comes to protection its the best ive found so far. Download and try the trial for 30 days and see what ya think?
Bit defender ive seen (havent personally used it) is also another one that is very good from what i have read and seen.
Free bees not to use:
AVG is a pile of crap, over and over again ive seen it not protect, stuff up PC's and slows down a PC greatly and well just be a pain in the ass, i wouldn't recommend it.
Microsoft Security Essentials lots of people think its good ive seen it on many many computers here ( the old computer guy that used to run a business here installed it on everyone's system) and i have found it to be no good, all of the computers that i have used with it on are just completely infected by viruses and in many cases have been disabled by a virus and there fore doesn't protect at all and lets a flow of viruses to come in. I wouldn't use it.
Payed ones i wouldn't use:
Norton is all talk no protection, countless machines ive worked on have stuffed up and wont work all because of Norton, it doesn't protect that well even Avast free beats the shit out of it. I will give it one good thing its no where near as resource hungry as it once was its ok NOW, but overall its not great at all and i wouldn't recommend it either.
Mcfee is "ok" but once again it doesn't seem to protect very well, i haven't had to deal with it to much around here but it doesn't do a great job, but honestly i think its better then Norton lol
Trend Micro seems to protect pretty good but it will make your computer 5yrs older, it just bogs down and slows up computers realy bad, all it does is read the HDD 24/7, i wouldn't use it, no good for a gamer.
Other programs i personally use to find viruses or malware is Malwarebytes and Hitman Pro, both you just run when you think there is a problem and these find alot of viruses and malware, there excellent, Hitman Pro is the best, dont have to install just click and run it even from a thumb drive, its brilliant.
Thats a quick run down of what i have found over the yrs working with many many computers with different issues not just viruses so i hope that will give you a better idea what to use and what to stay away from.