64-bit Windows since Vista has been very good at running 32-bit apps (in fact, I'm unaware of
any 32-bit apps that don't run on win7 64-bit). This is a change from the Windows XP days, where the 64-bit version of the OS was quite broken and incompatible. As such, calibre runs just fine on 64-bit Windows 7. In fact, aside from an old Atom netbook, I've never installed 32-bit Windows 7 on any machine
As a side note, most apps you run on Windows will either be 32-bit or CPU-agnostic. For example, every web browser you use will be 32-bit even on 64-bit Windows (IE and Firefox technically have 64-bit versions, but nobody uses them). This is because many popular plugins like Flash don't provide 64-bit versions. Want to see what I mean? On a 64-bit Windows 7 machine, search for "Internet Explorer (64-bit)" in the start menu (it should come up after typing "internet e" and run it, then navigate to a flash-using site like youtube. Boom! Warnings everywhere because no flash! Now start "Internet Explorer" instead (no "(32-bit)" tag at the end of it) and do the same, and everything will just work (assuming you've installed flash, of course). What's the point of this little exercise? To show that 32-bit apps run just fine in 64-bit Windows, of course.