My home PC has a virus, what can I do to get rid of it?

I think it's a really bad virus. The other day I was on this site that I shouldn't have been and an ad passed through my pop-up blocker and popped-up. I closed the pop-up window and immediately my Webroot anti-virus had a message pop up at the bottom of the screen stating "a potential harmful threat has been quarantined" or something to that extent. I closed my Safari browser (I'm using Windows Vista, not a Mac) and opened Webroot. The risk level of the quarantined item was 5 and it's name began with "MAL". I didn't write down the last part of it. Anyways, I didn't think much of it at the time. I thought Webroot handled the threat. I walked away from my computer and when I returned, there were a bunch of weird error messages popping up. I got freaked out and closed iTunes, then restarted my PC. When it restarted, there was a red circle with and "x" in it on the task bar next to the time, volume, etc. Also my desktop background was gone and it was just all black. Then a fake scan popped up on the screen and said that I had a bunch of threats on my computer. That's when I knew for sure that my PC had been infected. I force-shut-down the computer by holding down the power button. Then I turned it back on and tried to boot it into safe mode, this failed miserably as I was faced with a "blue screen of death". I turned the PC off again and I turned it back on and pressed F8 at the start up screen to view the advanced options. I had no idea what option to choose so I just felt discouraged and turned the computer off and I haven't touched it since Saturday when all of this occurred. I really want to get my computer working again before my mom realizes there's something wrong with it. Please help, thanks in advance.