Two of the best are
Spybot Search & Destroy (
The home of Spybot-S&D!) and
AdAware (
http://www.lavasoftusa.com/products/..._personal.php). Both are free to download and are fairly easy to use.
Tamlin, SpyBot puts adds into your PC to block other people's adds. They pretty much monitor you while keeping others away from that. It's also known to cause slowdown and erros.