Microsoft Office 2010 is the first version of office that will ship in both 32-bit (x86) and 64-bit (x64) flavors. If you are running a 32-bit version of Windows, you can only use a 32-bit version of Office. However, on x64 Windows, you can decide to install either 32-bit Office or 64-bit Office, but not both.
Microsoft recommendation is to install the 32-bit Office, even on 64-bit Windows. Unless you have special needs that require the use of 64-bit Office apps.