Please help...fake antivirus!!!

Last night I was on the net looking for a product key for a cd i have and i came accross a sight that my AVG scanned and said was fine so i felt pretty safe when i clicked a button saying "download product key". But as soon as I opened the icon that had been put on my desktop command promt opened and started doing things. Before I had a chance to say "oh S%!T" it made some funny codes then said "files copied". In a panic I hit reset. when my i got back to the desktop an "anti virus opened and started scanning then it said there was a few threats that needed to be cleaned but I had to register first. I'm an idiot so off I go clicking on the regiter button. It opened a site asking me to pay. AVG is supposed to be "freeware", it was only now that i realised the antivirus that popped up is fake. It is titled "VISTA Antivirus 2008"....I'm running XP!!!

Now my computer can only be used with this annoying picture coming up every 30 sec...saying that someone is trying to hack my computer remotely it even comes up after I pulled the net cable out.

The fake antivirus cannot be found in the "startup menu" of in "add/remove programs".

I'm not sure what other damage has been done.

If someone could help me figure out how to make my computer go back to the way it was before lastnight I would be forever greatful as it is new and cost me a LOT of $$$

Cheers,

Liam.