Infected

Hello,
okay i know nothing about computers,a few months ago i brought a acer aspire one
its tiny, and it doesnt have a cd drive.


earlier today i brought a shirt off a website :\
and about 10 minutes later a load of windows popped up
some anti-virus thing that came with the computer, i cant remember the name, m something. but yeah, it was trying to stop them i think, block this action etc.

then it froze so i turned it off and back on.
and now whenever i try and log on, it gets to the welcome iona, type your password part. but as soon as i log on the screen goes black and then returns to the welcome page.

I have absolutley no idea what to do, and the coursework on it is due in on monday, and it would be helpful to be able to just get some files off it or anything.

if anyone could suggest anything that would be brilliant,
thanks,

iona