This free program finds spyware and adware. These are little programs that advertising companies place on our computers. Adware feeds us ads; spyware reports back on our browsing habits.
These programs invade your privacy. And they will slow your machine. So it's a good idea to clean them out.
Lavasoft's Ad-Aware is a good program. You should use it in conjunction with other anti-spyware programs. That way, you've got all your bases covered.
How often should you run Ad-Aware? I run my anti-spyware software a few times a week.