One easy way to block ads from appearing on your computer is by modifying your Windows “hosts” file. This file allows you to specify an IP for a specific domain so any requests from that computer will be automatically sent to the specified IP.

For instance, a line in your hosts file which looks like this:
google.com 127.0.0.1
will cause any attempt to go to Google to be force to look to the local IP (127.0.0.1).

You can create entries for ad servers here as well, essentially blocking them. The problem is there are literally thousands of ad servers you would have to find and maintain. No worries, simply download and install Host Secure from this site:
[http://www.timdorr.com/syko86/ycsoft/hostssecure.htm]
and allow it to automatically maintain your hosts file for you.