Press control+alt+del and read the list of programs running in the background. If any of the following are listed, you've got adware secretly installed on your computer, collecting info on you and broadcasting it to some slimy buttwipe over the internet, hijacking your internet homepage -- perhaps even logging your keystrokes and stealing your passwords, email messages and everything else:

Mmode
eZula
Text iLookup
Bargain Buddy
Iexplore
Save
deals
Gator
... many more

Also look for folders with the same names in your "Program Files" directory. Another way to find them is to click "Find Files and Folders" in your Start menu. Some can be removed using the "Add/Remove Software" function in the Control Panel folder, others simply have to be manually deleted from the "Program Files" directory.

I found formatting the hard drive very effective. And whenever I installed new software that was originally downloaded off the net I checked it for new adware that might have come bundled with it.