help! my computer is infected!

Hi,

My computer got infected about an hour ago and I have tried doing multiple things to get rid of the infection. I think the infection is called system security version 4.51 because it randomly installed on my computer and I have tried to uninstall it only to find that there is no uninstall button! Also, every time I try to get into my system, it will not let me access it and keeps making pop ups apear saying that I have all sorts of viruses after completing a fake scan, and that I need to download the program because someone is trying to access my credit card information. When I try to download help to get rid of the virus it will not open and won't let the program complete. I have a windows xp. Nothing seems to work!

Thanks so much!