First thing I recommend is that you download: (1, 2 and 3 if you are an expert, 2 and 3 if you are an average user)
Microsoft Security Essentials
Spybot Search and Destroy. (Also, from download page, get the latest definitions)
Now, keep all these files in a directory or on a USB stick.
Look at your hosts file -
c:\windows\system32\drivers\etc\hosts and make sure there are only one or two entries for localhost, or if there are more and you know why, leave it.
**(If using internet explorer) ** Next, go to internet options and in the last tab, click
Reset Internet Explorer Settings. This should remove all toolbars and any "bad" settings.
Boot in to safe mode and install Spybot Search And Destroy, let it run and remove all the rubbish.
Next, install Microsoft Security Essentials.
Lastly, if you think you know what you are doing, use Autoruns and check Windows Startup and get rid of all the extra entries (if there are any... or just give your computer a spring clean).