You should immediately download and install a resident antivirus. I can recommend a free one called AVG, which you can find here:http://free.grisoft.com/doc/1
If necessary, download it to somewhere you can find it, and install it in SAFE MODE (tap the f8 key whilst booting your computer, you will get a funny screen which will have the SAFE MODE option). Once installed, be sure to UPDATE the definitions on line (you can do this from the AVG control panel). Have AVG scan your hard drive and then get rid of everything it finds. Keep it active and keep it updated; to rely on on-line scans is closing the barn door after the horse runs out.
Once you have completed this, then you may be able to download and install two other good applications that will check your computer for adware:
Spybot Search and Destroy
These three applications will, more than likely, rid your hard drive of a LOT of nasty things. If, after doing this, you still have problems, post back.
You will find the links to the adware applications, and some good basic steps to protecting your computer at this tutorial:http://www.bleepingcomputer.com/tutorials/keep-your-computer-safe-online/