I hate these things... never ever click the links shown when you see "your PC is infected" on a web page or pop up box. Even trying to close the window will install the program, which is usually virus like itself. If it does a "scan" it will say nearly all your important Windows files are infected and the only way to remove them is to use their program.
I've cleaned a few PCs of family members that were infected with these sort of programs and it's not a quick process, but the following may help.
If you get this sort of thing, either ctrl alt del or ctrl shift esc and end the browser window using task manager. Don't click anywhere in the browser window.
If you are infected, restart in safe mode with networking, download and install Malwarebytes (
http://www.malwarebytes.org/, you may have to use another PC to download it to a flash drive), run it on full check (will take forever) and do what it tells you to do. Then delete all restore points (turn system restore off then on again). I also run CCleaner and HiJack this to see what else is starting up.
These things are the scourge of the internet and prey on the biggest weakness in any PC's spyware / virus defence - the person at the keyboard.
Good luck