Download a program called "Spybot: search and destroy" or "Ad-aware SE personal"
Run it and it should clear up any problems, also take a look in your add and remove programs list for any suspicious programs, for example I had an annoying program called "Spydoctor" on my system, and it had been installed onto my hdd, i simply uninstalled it and it was gone.
Hope this helps
My Setup -
AMD 64 3200 +, 256 mb Geforce 6600, 1GB PC3200 DDR400 RAM, 160GB SATA 7200RPM HDD