Popups are those annoying ads that pop up (or under) while you’re surfing the ‘net. There are three kinds of popups, and you’ll need to do three separate things to block them.

For Windows Users:

• Pop up windows created by web pages can be easily blocked by a third-party pop up blocker like the free Google Toolbar. However, the best way to block popups (and other nasties like browser hijackers) is not to use Internet Explorer at all. I recommend Mozilla or Firefox. Both have built-in pop-up blocking but you must turn it on in the preferences.
• Pop-ups can also be created by spyware. These programs run in the background and can appear any time, even if you’re not online. I recommend Spybot Search & Destroy to clean out spyware and block it in future. If you’re going to continue to use Internet Explorer also get Spyware Blaster to prevent browser hijacking.
• Finally, there’s a really annoying kind of pop-up that’s, in my opinion, a security flaw in Windows NT/2000/XP. These versions of Windows have a function called netsend that’s designed for system administrators. They can use it to communicate with all the computers on the network. Unfortunately, it can also be used by a spammer who can send a pop-up to ANY computer on the Internet. To block this form of Messenger spam you can turn off the Messenger service, or turn on a firewall. Enabling the built-in Windows XP firewall will block this spam forever. Windows XP Service Pack 2 turns on the firewall by default.
• The best popup stopper I found is Pop-Up Stopper. You can overide by pressing ctrl. It kills all of them and it is free! http://www.panicware.com/product_psfree.html -Buzzrr

For Macintosh Users (by bruceb):

• Pop up windows created by web pages. In Safari, ensure you have “Block Pop-Up Windows” selected in the Safari menu (there should be a check mark in front of the item in the menu). Other Mac browsers vary, but there’s usually a setting somewhere in the browser’s preferences to turn on popup window blocking.
• Pop-ups created by spyware. So far at least, Macs don’t get spyware, so this just doesn’t happen on Macs.
• Messenger spam. This also has no Mac equivalent; it just never happens on a Mac.