When I read this thread, I felt the other response needed a bit more detail, so I thought I would reply.
Technically, 64-bit IE cannot be your default web browser in Windows 7. But it is possible that IE is just not set to be your default.
First step is to verify that IE is set to be your default browser by clicking theMake Default button in IE > Tools > Internet Options > Programs.
You will notice that 64 bit IE is not available to be the default. (see image)
If your 32-bit IE is actually broken, I would recommend making sure that you are on IE9 and upgrade if not (IE > Tools > About Internet Explorer). If you are already on IE9 you can try looking at the Microsoft forums for how to fix your specific issue or taking you computer a trusted repair person.
But as the original responder stated, you can always install an alternate browser and set it to the default on install or via its preferences/options section and that should also fix your issue. If I were to select an alternate I would choose Google Chrome, though Firefox is still quite good, and there is always Safari, or even Opera.
Good Luck