internet explorer opens up firefox wth??

ok.. my problem goes like this...
whenever im in internet explorer, and click on say, home or one of my bookmarks, it opens up firefox and opens up the page there. why on earth is it doing this?
when im in windows live messenger and try to open my email from there, it'll open up internet explorer, but give it a blank page, and then continue to open up the page in firefox. >>>.<<<
if i click a link that's on the webpage, it'll open up as normal.

i've had this problem before, and so i did a repair installation and it fixed it, but i'm not prepared to do that again, especially if it's gonna be happening again and again. anyone got any ideas of why this is happening?