Avast! Home edition is free from Alwil Corp (their Pro is pay for). I swear by three things. Spybot S&D, Avast! Anti-V and Spyware Terminator (caution, last one is not compatible with Win Vista but is with XP Pro and Win 7).
I also swear by CCleaner for the clean up of the registry - another caution DO NOT play with your registry if you are unfamiliar with how to tinker with it. In the registry (getting to it is "Run" type "CMD" then in that command prompt type "regedit" or just "run" & "regedit"). CCleaner takes most of the guess work out of getting rid of viral\malware registry files (you HAVE to get rid of these to keep the virus from replicating or re-replicating into your system).
Another thing I do is drop all my Restore System check points by going to the "monitor drive" tab in the control panel and unclicking the monitor drive, it dumps the restore check points then I re-click monitor drive after I do all my anti-v and maleware scans. I also do them in Safe Mode rather than regular boot up (You get to safe mode by restarting the computer, hitting F5 or F8 depending on operating system, choose Safe Mode with networking or without - then do scans). What that does is disable every unessential driver except windows drivers (it also disables the virus DLL files or driver files that are being over taken by the virus or malware). A good indication of malware is checking your send and receive on your network status. (right click, properties). If the send numbers are larger than the receive - you can bet your bottom dollar you are infected with a virus or malware. You are losing what is called "packets" of digital information when you send out and then receive back. It's because the virus is sending itself out there and running up the bandwidth on the receive side, causing the modem to become stressed and slow...the symptom of that is losing digital packets of information out in the WWW.
Another thing I do with my anti-v and anti-spyware software is update update update. Spybot S&D does not auto update, neither does Spyware Terminator (open source=free but not always full of those auto updates perks, has to be done manually and these two software are worth it). Another thing, ALWAYS do the windows security patch updates, at the minimum. Weekly, set your auto updater to do it if you don't remember to do it. Security patches are essential in keeping hackers, malware and viruses OFF.
Any other questions I'll be glad to answer and help where I can!