worm

In the past there have been several worms and viruses that block websites. Conficker is included in this list: it blocks access via the web to several major antivirus vendors' websites.

This means that there is a simple, web-based test to detect its presence:

http://www.confickerworkinggroup.org/infection_test/cfeyechart.html.

If you see all the images, you're not infected :).

I used to be able to run windows without antivirus software, but not any more in the days of the crapware that spreads by itself...