Follow these steps:

-- Make sure you have Antivirus and Antispyware install and it's fully updated.
-- The best antivirus programs are:

-- Norton Antivirus (http://www.symantec.com)
-- MCafee (http://www.mcafee.com)

If you are looking for Free Antivirus program then you can go for

-- AVG Personal Edition (http://free.grisoft.com)

Free Antispyware is Ad-Aware SE Personal Edition (http://www.lavasoft.com)

Ad-Aware Personal provides advanced protection from known data-mining, aggressive advertising, Trojans, dialers, malware, browser hijackers, and tracking components. This software is downloadable free of charge.

If you are Advanced user, then you can use this free utility to check the PC infected by virus.
Download this free utility (HijackThis.zip - size 208K )