For those that do not know, Microsoft's paid security software, Live OneCare, went bye bye in early 2010 to make way for the free Security Essentials (you must delete the Onecare software from your computer to put on the new software). Absolutely free, it trumps Norton Antivirus and McAfee in several ways: it's free on XP, Vista, and Win7. It is updated twice daily. It is non-intrusive. It covers all forms of malware and viruses. Did I say, it's free? Oh, yes.
Take a look at it--just Google Microsoft Security Essentials and find the microsoft website and downloads in about three minutes. Who better to keep spyware off your PC than Mr. Gates himself.