Help!

It appears I have a virus.

I cannot run or install antivirus software.
Windows task manager no longer appears when I press Ctrl-Alt-Del
I get occasional pop-up internet explorer windows (Which is strange as i use firefox)

Can someone help me?

I would suggest using Knoppix to troubleshoot your situation as well.  I hope you are still able to download the image file and burn it onto a CD though.