Computer trouble, please help!?

I have recently downloaded (not on purpose!) a trojan/virus/ something that I don't want. It flashes up every 10 seconds saying that my computer is infected. If I click it it starts downloading a fake 2009 XP spyware program. I need to remove this because it is causing my computer to be extremely slow. I tried to open up Norton, my spyware scanner, however, it won't open and says 'abnormal program'. So, I thought I'd try downloading a few free spyware scanners - such as AD aware and AVG. However, these scans all stop working - presumably because of the 'thing' I downloaded. Also, it won't let me on websites to download the spyware scanners - I can get on every other website. I really need some help to get rid of this, because it could really mess up my computer. Can anyone help me?