These days you need both anti-virus and and anti-spyware software on your computer. Yes you can buy it but the commercial programs have gotten so blotted and huge and add so much overhear to your machine it's sometimes not worth the effort.
Here are some of my suggestions for both type of protection. You really only need one anti-virus but running all the anti-spy ware programs is recommended.
Microsoft has released a new Anti Virus/Anti Malware Package that is free to valid Windows users
Microsoft security Essentials provides protection is automatically updates using the Windows Update Process and initial impressions are that it places less load on your machine than other available protection packages *** My Personal Pick***
AVG They also have a commercial version but freebie works just fine.
Avast Again they have a commercial version but this works great
Ad-Aware The Personal version is available for free.
Spyware Blaster Also a freebie and works proactively to keep you safe.
Spybot Search and Destroy Great tool and again it's proactive
Windows Defender A great anti spy ware tool and the price is right, you just need to have a genuine copy of Windows
Like any program these are only any good if you keep them up today and do a periodic scan, particularly with Ad-Aware and SpyBot Search and Destroy.