anti-virus scanners1. One of the best free antivirus scanners that I have come across is easily Malwarebytes anti-malware from -> www.malwarebytes.org/ If you have not got this on your computer I recommend heading over to their website and download it right now! The only downside to this program is that it has no run-time scanning (this is the free version), it has to be manually updated and set to start scanning.
2. Another great free antivirus is avast. It is a good all-round anti-virus with runtime scanning (scans files as you use them) it also automatically updates and has a gaming mode so it does not start scanning your computer/downloading updates while you have this option ticked.
MalwareBytes+Avast+Windows firewall is a great combination for all of your antivirus needs.
One more Extremely useful tip is that if you ever download a suspicious file eg: 'CSS HACK 2.4' you can use an online scanner such as -> http://www.virustotal.com/ to upload your file to a server that will then scan that file with many different antiviruses to see if it is infected or not. You will then be given the result.