Try running some freeware called "Ad-Aware". This software will scan your computer similar to virus scanning software and then will allow oyu to remove almost all unwanted material. To find it, do a search on the web for ad-aware and you will probably find to vendors with this software available for a free download. Some intrusive ad software will hijack your browser or block you from obtaining ad-aware as a way to keep you form cleaning out thereillegal downloads. If this happens let me know and i will send you a copy vis e-mail.
Sometimes you will have some more mainstream site material on your computer that mya not some off and then you will have to attempt action similar to Suzie's post, however, i would not mess around with the registry unless you are quite coimputer literate. i can help oyu via IM or e-mail with that process after you have run "Ad-aware" if you still need assistance.