Download and try to install the AV, some viruses will try to block the install - but it can't be worse than it is now. In the event you can't install it, then you'll have to get a rescue CD - or maybe as a USB drive if you have one of those netbooks with no optical drive.

There must be dozens of reputable AV suites available, free programs like AVG, Avira and Avast or commercial versions including all the above (they have both, and will try to convince you to upgrade to the paid version) or Norton and MacAfee and so on. Use something reputable, not just something you see an ad for - some of the ads are malware that pretends to be an AV suite. Just make sure it says it will run with 8.0 and 8.1.