The desktop version of Internet Explorer 10 tabs in 32-bit by default, but this can be changed. The “Modern UI” version, or whatever it’s called now, runs in 64-bit mode all the time. However, both versions run the browser frame process in 64-bit, and the desktop version runs tabs in 32-bit by default for the sake of plug-in compatibility. Since the Modern UI version doesn’t support any plug-ins at all, Microsoft decided to make it all-64-bit.
If you want to run the tabs of the desktop version in 64-bit, too, then you need to turn on Enhanced Protected Mode in Internet Options, Advanced. Don’t be fooled by the ticked box next to Enhanced mode under Security. If you didn’t tick the box under Advanced, it has no effect. In the RTM version there no longer exists a separation like there was in Windows 7 in both the Start menu and Jump List. The only difference is the above-mentioned setting. Regardless, in the Task Manager, IE 10 will always appear as a 64-bit program.
Bear in mind that if you run Internet Explorer 10 in 64-bit, most plug-ins won’t be compatible, but you get more strict security in return. There’s a 64-bit version of Adobe Flash available though, but ironically not of Microsoft’s own Silverlight. To this day, there is no word from Microsoft on whether IE 10 will be available for download on Windows 7.
If you want a much more in-depth and technical explanation of the Enhanced Protected Mode, then head over to the MSDN blog.