Clean software for Windows machines
First, clean the machine with one, or better yet two, adware cleaners. Ad Aware is a good place to start. (google for it)
How to practice safer computing
Then make sure you don't install known adware/spyware.
Don Davidson's list with links to removal tools
Then go get some clean, free and cheap sofware at cleansoftware.org
Don't forget to pick up the free antivirus software while you're there, install it and turn it on.
Why do I post so much about spyware and antivirus tools? Because I know many of you are using computers that are slowed to a crawl by it and that others have a computer that's been completely crippled to the point it won't work anymore.