why does ad.yeildmanager.com/imp%3Fz pop up all the time while I am looking at stuff?

You have not got a pop-up blocker. I really recommend this one - I think its one of the best!

http://sumstuff.ourtoolbar.com

You can download the Yahoo! Toolbar that is free and contain pop-up blocker to block this.

You have spyware, my friend!

Download spybot and adaware from http://www.download.com/ and check your pc for spyware with both (and remove it of course) and you should be good.