|Since you still haven't said which you're using, I'm going to assume IE7.|
Go Start -> Control Panel.
Make sure you click on "Switch to Classic View" in the upper left if it's there. (If it says "Switch to Category View", just leave it alone. You're where you need to be.
Double-click on Internet Options.
Click on the Advanced tab.
Click on the Reset.... button near the bottom right.
You should get an "Are you sure...." window. Click on Reset again.
This should reset all your cache, cookie, forms, etc. settings. You should not have a problem logging into the site after you do this. Post back here if you do. I'm going to be gone for a few hours, but will look back here when I return. Should be around 8 or 9 EST.