When people start talking about pop ups, they need to be more specific. There are a few different kinds, including pop ups stemming from a spyware downloaded and installed on your PC, active x pop ups set to do so from a webpage, windows messaging pop ups, a few others. SOme things like a firewall will help (XP firewall is ok, but a cheapy internet router with a hardware firewall is better). Some things you just have to avoid, like when you go to a webpage and it asks if you want to install XYZ software to view the page, or software to help you download things faster from the web. JUst don't say yes... that's all.

Viruses usually come from manually opening something, whether it is something sent to you in an email (VB script, executable, zip file), or something you thought was legit that you downloaded to install, and it happened to be infected. Sometimes you get a "pop up" from the virus, but usually it is only to laugh and make fun of you for infecting yourself, then it runs and really screws you.

Spybot, Adaware, etc won't save you from everything. It will help get rid of some spyware and other "dialers" and things, things that aren't too harmful... but viruses can be avoided by having a good virus scanner, and scanning anything in email or that you download before you install it.