If you call this 800 number you will get someone who's job is to get you to let them on your computer so they can scare you into paying them to clean it up. EVERYTHING they say is a lie and the only thing they want is your money.
The page that pops up on your computer is, usually, very hard to close. Just restart your computer and 90% of the time everything will be fine. If you are one of the 10% that still have problems don't call the number on your screen. They will try to charge you 2-300 dollars to clean what they put on your computer in the first place. Call a reputable computer company. Of course my choice would be for you to call Computer Solutions but if not just make sure whomever you contact has positive reviews or a solid reputation.