To make the change, you'll need to reset your system's DNS numbers; if you're using Windows XP, use AdBarricade's downloadable program to do it for you. (If others are on your network and want to use AdBarricade, you'll need to manually make the change through the router.) Make sure to download AdBarricade's small auto_remove tool, just in case the new DNS settings prevent you from accessing the Internet.
AdBarricade does a terrific job of killing banner ads, and a reasonable job at blocking most Flash ads. Unfortunately, the service had no effect on pop-ups, pop-unders, or fly-ins, so you'll still need to use a free pop-up blocker. You can give AdBarricade a workout for seven days; if you like its performance, it'll cost you $19.75 per year.
Note: this download takes you to the developer's site.