using XP + internet explorer? SP2 adds pop up blocker to IE6.
or
google toolbar for IE
or
use a better browser that has had pop up blocker for ages: opera, firefox etc.
or
before someone else says it: i dare say macs don't suffer from this problem as they are apparantly perfect in every way.
you'll be happy but seriously skint
B.) If you won't listen to the voice of reason, then pays your money down and get Lavasoft Adaware .....actually I think the base version is free to individual users.
Does what it says on the tin at the expense of a small processing overhead. (And you are still motoring on the dark side.)
Firefox solution is the sound one. Less mess, more light.