You prolly have a bunch of shit running in the background running amock. Maybe even might be a problem with JRE on some pages.
You can downgrade back to IE6, it's easy to do once you find the steps. I think you can do it in control pannel.
Clean your shit up and check running processes. Obviously IE runs fine right out of the box, but when it's fucked with (ususally by viruses/adware) it don't run so right. Firefox may run a bit better, but if you have crap eating up resources you've still got issues.