You have gotten some very good advice! Most of the free programs are "cutdowns" from a paid program, and only do part of the job.

But you can't blame the programmers for wanting to get paid for what they do!
AdAware by Lavasoft is probably the best of the best out there, followed closely by Spybot.
Be very careful with HijackThis! It puts you into the registry, and if you get too enthusiastic you can totally mess up your programs. If you are pretty computer knowledgeable, go ahead, but be prepared to re-install any or even all of your programs. As my dh programs in BAL and is a certified "knuckles drag the ground" computer geek,

I don't worry about it too much. But even we have gotten into major problems getting too deep into the registry entries for a program from time to time!
You might want to try out Vopt from goldenbow.com - they have been around for decades, and Vopt is probably the best of the best hard drive defragmenter going. you can download a full-blown trial version that expires in about 30 days. After that it's about $50. I have it on all my computers and wouldn't be without it! What's really funny, is that we got this program, paid for it and all, and were totally blown away with it. Then a few months later I went to a college reunion and hooked up with one of my old friends and it turns out she and her husband "do" Vopt! She was one of my best friends, and we lost touch with each other over the years! We ended up going out to visit them in CA a couple years ago! Small world! But that doesn't take anything away from their program, I assure you!
I also recommend checking out grc.com. Steve Gibson has been around doing stuff with computers since the 70's. He writes all his programs in BAL and they are very small and run trouble-free. He makes many utilities, and many are free. He wrote one of the first anti-spyware programs (OptOut). Check them out.
Cheerio!
Elizabeth