I believe the problem isn't on the site. An adware must have infected your computer. Sometimes when you install a software there're adwares that are installed together if you choose the typical install (always choose "custom install - you can see what is being installed and accept or reject it). They can show as a toolbar in your browser, but sometimes they don't show at all. Adwares often do what you are describing.
I suggest you check your computer for virus, adware and spyware. There're free softwares and online scans that can find these malwares and get rid of them.
Andreia
