I work tech support for UofM and I get this kind of question ALL the time. Popup's are annoying but I think the BEST way to prevent them is to keep your computer free of adware and spyware. "Popup blockers" are totally unnecessary if your machine is clean.
I get a few a day but nothing like the machines I see with spyware. (for example...
www.weather.com has one ad that pops up everytime you visit their site. That is bearable to me... i just close it and nothing else opens).
Download and install ad-aware 6.0 and run it on your machine. Delete all of the files and registry entries that it comes up with. I have done this on HUNDREDS of machines and have never had a resulting problem from it (windows 2000 and XP only though... I don't know about win95 or 98).
DO NOT download and install programs like Bonzi Buddy, Webshots, Weatherbug, Xupiter, Hotbar, any toolbar cutomization program, any "faster" searches program, any "make money by surfing the web" program, any coupon program etc... These ALL contain spyware/adware and will increase the amount of popups you get.
Installing a popup blocker may stop the popups but it doesn't resolve the underlying problem. Spyware and adware report back to their programers the types of sites you visit so that they can send you more and more popups!
HTH!