Just implemented this and it kicks ass. Recommend it highly:
http://www.mvps.org/winhelp2002/hosts.htm
Basically what it does it augment your HOSTS file to redirect all the major advertising domains to your loopback address. What this means is that when one of these sites wants to show you an ad, it redirects the source to what's on your computer, most likely nothing.
No more Youtube ads creeping up, popups, or interruptions. I do have to say that it does limit access to sites like serials.ws, or that, for example, National Geographic streaming videos will get stuck, but you can remove this. Post here if you have any questions.
