Free Antivirus & Antispyware
The most useful and updated antispyware software available is AVG Antivirus Free Edition. Click Download Now to download the program from Download.com. Be careful to use this link since there is also a fee based version of AVG and it will bug you to pay for updates, removals, etc. ALSO, take care to install this tool, get any virus definition updates, THEN DISCONNECT from the internet to run the program. Restart, rerun, WHILE DISCONNECTED, and restart a second time.
IF you suspect a virus or spyware and can still get onto the Internet, or are doing regular spyware checks, DO IT WEEKLY, then another great system that does not require an install and is VERY updated with new virus definitions is Housecall by Trendmicro. It will scan for both spyware and viruses and is updated very often. Housecall is best for periodic weekly maintenance since a bad virus might send you to a vicious Website and not the Housecall Web site. See the following screen shot image to see exactly what the Housecall Web page looks like...

After you are cleared of viruses you should also run a spyware program since some will be missed by the virus software. Spybot Search And Destroy is the best available with regular updates for free. Click download Now. AGAIN, follow this link, or burn it from an uninfected system to a CD, GET UPDATES online, then GO OFF LINE to run the program. Restart, rinse and repeat. No don't rinse anything! Everybody's a comedian, just not good ones!