The worst possible Antivirus programs that you can install on your computer are Mcafee AV and Norton AV. Mcafee AV is a very ineffective virus scanning program and its virus detection is very poor. Norton AV is an ok virus utility, except that it consumes a TON of system resources to run.
If you want to purchase Antivirus programs, you might consider Kaspersky 6.0 or Nod32.
Also, Panda AV is actually a pretty reliable AV program, although some people have trouble with this software at times. I personally use Nod32 since it is one of the best Anti-Virus utilities out there; great virus detection and almost zero use of system resources while still having active scanning.
If you don't want to spend any money on an AV program I recommend looking into these free AV programs: Avast AV, Avira AntiVir, AVG.
Avast is a really great free AV program. It detects quite a considerable amount of virus infections and other various worms/trojans, consumes little system resources and has an effective real-time monitoring system. It also has a nice feature of running a full system scan when your computer starts its screensaver.
Avira AntiVir is another higly effective antivirus program and has great realtime protection, low system resource consumption, and high infection detection capability. The only drawback is that a 'nagscreen' pops up telling you about its full version everytime your AV definitions update. But all you have to do is press ok on the bottom and it goes away. There is a way to disable this, but it requires a little work.
Then there is AVG Anti-Virus. This program is ok, takes little system resources, but it not as an effective an AV as the above two mentioned (virus detection not as high).
Along with them, use Spybot: Search and Destroy, Ad-Aware Anti-spyware: Personal Edition, Super Anti-Spyware, and AVG Anti-spyware; all very effective infection cleaners.
Then to clean your computer even further, run ccleaner, run cleanup on your HD and then your registry. Download "KillBox" and go to 'Tools' and run 'Delete Temp Files'. Run the cleanup on each profile in your PC.
After cleaning your system fully, you might consider using Hijack this and check out if there is anything else that is suspicious running, since Hijack can see things that a lot of utilities can't. Or you can even use a very nifty program which is practically better than hijack this, called RunScanner, found here:
http://www.runscanner.net/
When your done, clean up your startup items in msconfig and remember to clear your system restore points and create a new one to the point of where your system is nice and clean.
Also, there really isn't any reason to install a third party Firewall, especially if your behind a router. Using Windows built in firewall is effective enough, as it consumes a lot less system resources than any other installed program.