Internet Explorer turns into AOL

I use Firefox and Thunderbird. My OS is Windows XP Home, my computer is HP a1550y.

I am unable to access Internet Explorer, although the icon is on my desktop. Every time I click on it I get to a window that says Internet Explorer but is actually AOL. I have tried 3 times this evening to download IE 7 but every time I click on the new icon I am asked if I want to run this download, and it goes through the whole download/upgrade process, but doesn't get me to IE itself.

For some reason no one has ever discovered this computer has never accepted IE upgrades, etc, but until now I could at least access IE.

The worst thing is that on Thursday (!!!) I have to teach a beginner's computer class, and will have to deal with IE on our computer room machines. In another week I will have to teach email, and I haven't a clue.

Help!