(And to continue the thread of adding another program to the list)
Download and run ‘Adaware’, ‘Spybot Search & Destroy’ AND check for viruses. (I recommend Trend Micro’s scan initially http://housecall.trendmicro.com and then run another free online checker/remover afterwards (try Panda software’s solution))
Usually a combination of the above 3 will be causing your problems and tonnes of people have had exactly the same problem as yourself.
This is not a foolproof solution and will not permanently fix your machine - it will just get rid of the current problem. Removing everything to ensure that your computer is free of all future problems takes a bit more time and effort.
Good luck! Let us know how it goes.