I have a few things to say on this topic... so bear with me.
First of all, there's an imposter out there. You'll probably even see a Google ad on your screen right now for "SpywareBot Free Download". That is NOT SpyBot!!!
In fact if you do a Google search on "SpyBot" or "Search & Destroy", the top listings will also take you to this fraudulent site. They're obviously paying Google enough to keep their listing at the top.
If you download the bogus product it'll scan your computer, allegedly identify about 600 problems, then take you to a registration screen where you have to pay to have the problems removed. This is so illegal on so many levels in so many countries that I can't believe it's happening.
In fact I consider it ironic that these people are using spyware to sell spyware!!!
I found my correct Spybot download at Download.com.. but be cautious... there are ads for the imposter there too! If you're on the correct Spybot page at Download.com, you'll find a video explaining the pros and cons of what is generally a good, free product.
I also use Ad Aware and AVG Anti-Spyware (both have free editions). I find that each of them will uncover problems that the others have missed.
Now, this is obvious to most people, but don't confuse anti-spyware or anti-malware products with anti-virus products. They deal with quite different problems. Only some of the commercial software security packages will tackle both problems.
If you want some basic info on computer/Internet security, along with some suggested commercial products, go to...
http://Confidential-Transactions.com
As you may expect, you get what you pay for, so the free products will only take you so far. At that point you have to decide what computer security is worth to you.
(BTW.. someone else here criticized Ad Aware because it finds and removes infected System files. I consider that a GOOD thing. A contaminated file is a contaminated file! If it means re-installing system files from your Windows CD later, that's fine.)