Opinions differ geatly on the matter, but I and many other find Avast! to be better than the expensive packages out there.

It is free for personal use and can be downloaded Download

You do need to register (free) to use it, the program helps you do so.

For spyware there is the golden tool, spybot search and destroy
Download

Lavasofts ad-aware fights advertisement bots (adware)
Download

Microsoft Windows Defender is another tool some use, I had no luck with it. It didn′t pick up anything that spybot didn′t, so wait for someone elses verdict on that tool.

THe ones I have linked to are all rock solid when used properly.