My computer came with the full version of Webroot's Spysweeper installed, and it's a superb program.
Previously I had relied on AdAware and Spybot but although these two freeware programs are good as far as they go they fall a long,long way behind Spysweeper.
One advantage is that it's on all the time-it will even block access to sites where it knows there will be spyware (this block can be removed at your own risk)
The other thing I do for both spyware and virus protection-and this is available free for all users-is NEVER,EVER use Internet Explorer for browsing.I use Firefox but there are other good browsers available too.It's also a lot more capable and adaptable browser.
I've seen one security expert state that Firefox without virus/spyware protection is still safer than Internet Explorer with.