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You have failed to use Common Sense, and now your computer is infected. What do?
Alternatively: Wipe the entire system, reformat the drive, and reinstall your operating system. Now, why do we advice you to go to such lengths to get rid of malware? The reason is that once you are openly infected, there is no way you can ever be sure you still aren't harboring some kind of nasty botnet (the real kind, not the meme /g/ kind) even if you seem to have gotten rid of the most obvious symptoms.
But if you must...
If you don't want or can do the above, do this instead. If there are any toolbars or search add-ons listed in Programs and Features, try to uninstall them from there first. Sometimes that's enough to get rid of it.
Check your list of browser extensions and add-ons, remove anything suspicious, sometimes adware will install as a "managed" extension and your browser probably won't let you remove it, you'll have to delete the folder where your browser is installed and then reinstall.
Also run MSConfig and disable suspicious startup items that aren't supposed to be there. (Google the name if you're suspicious.)
If none of these fix your problems, try scanning with Malwarebytes Anti-Malware, Clamwin, and if that doesn't fix it, try TDSSKiller, ADWCleaner and F.Secure Blacklight.