How to get rid of pop up ads?

I've never had a problem with pop up ads before, but just tonight, every time I go to a new page one pops up.
Is there something I cad do to fix this?
I have Windows XP

the firefox popup/popunder blocker isnt the best in the world...

you deff have block pop ups ticked?

there are add ons for FF

loads of user made popups blockers but I dont know how gd they are as I've never used them

try adblockplus dot org

Are you using Internet Explorer?

firefox

Try this:

-Go to Tools in the menu
-Go down to the Options button,click it of course
-Once the window opens up,go to the Content tab
-If the check-box for Block Pop-Up Windows is not checked off,click it so it is checked off
-Hit the OK button at the bottom of that window to save your options

It was already checked off.

adblockplus will get rid of *most* ads.