Spyware is the most persistent problem for computers today. Spyware is a small program that gets on your computer through downloads or by going to unscrupulous websites. The websites I refer to can be as simple as Goggle.com instead of Google.com. A single misspelling can fill your machine with junk.
Now why is that so bad? Well, at the most benign level, these programs are badly written and will cause your computer to crash, or increase the amount of pop-up ads or spam to your email. Unfortunately there is a far darker side called Malware. Malware, think malicious, tries to get personal information about you: your name, address, social security number, and credit card info. Now understand that Spyware is not a virus and cannot be stopped in the same manner.
I recommend a free program called Ad-Aware. Ad-Aware cleans a computer of spyware. Run this program to find all the little nasties and delete them. Also there is a podcast for those readers who want more information. Remember to keep this program updated. Give it a try!