LDH, Here is a free download for a utility that jimlur and others here turned me onto called "spybot". It is not a pop-up blocker as such, but it removes the spyware and cookies etc that websites place on your pc that cause a lot of the pop-ups. Since I downloaded it and ran the program it has reduced the pop-ups significantly. Here is a link to the free download.
Also, I use a free pop-up blocker from panicware.com, they have pay for versions as well...but I haven't had to use it much since I installed and use spybot. Here is the link...
Another good one was recommended by corp_buyer I think, it is webwasher. It is more powerful than the free version from panicware that I am using, but requires more computer literacy as well, LOL, hence I couldn't get use to it. The drawback is that it might block websites or certain email (like my wife's hotmail, LOL) that you don't necessarily want blocked. You can prevent this by listing the sites you don't want blocked. Here is the link to webwasher...
Linedrivehitter, since I installed the google toolbar, which has a pop up blocker in it, I have had no pop ups on any of my computers. Its free and with the toolbar you also get a google search option on every screen of internet explorer. I think it is great and takes up minimal resources.
Go to the main page of Google then select services and tools, then scroll down to google toolbar. I believe the following link will bring you directly to the google tool bar download page. If so, just download and follow the directions. There are various options when it is installed. I recommend it to all.