How to force Windows run only 32bits IE8 rather than the 64-bit?

"You should have ""Internet Explorer"" and ""Internet Explorer (64-bit)"" under Start/All Programs. You may examine these shortcuts by right-click and properties, to find how to run each version.

The path to the 32-bits version is probably something like this (I can't verify, not having a 64-bits system):
C:\Program Files (x86)\Internet Explorer\iexplore.exe

You can verify that the right version is running in Task Manger, where the running 32-bits process should be shown as iexplore.exe*32."