How to keep malware, spyware, and abusive Flash animations off your computer.
Find the HOSTS file. If you don’t know where it is, use File Manager to search for HOSTS. Ours is at C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS
Edit the HOSTS file (Notepad will do; you have to tell Windows which program to use because there is no file extension) as follows. The indicated domains are banned from contact with our computer; 127.0.0.1 appears to black-hole them.
More here: http://www.mvps.org/winhelp2002/hosts.htm