Office 2010 is the first version of Microsoft Office to be available in 32-bit and 64-bit versions.
If you are running a 64-bit version of Windows, you should install 64-bit Office, right?
Official advice from Microsoft is to use the 32-bit version unless you specifically need to make use of some of the capabilities only available in the x64 version such as Excel 2010’s support for larger data sets.
The problem is that Office 2010 x64 introduces breaking changes that mean 32-bit add-ins may not work in 64-bit Office. For a complete discussion see http://msdn.microsoft.com/en-us/library/ee691831(office.14).aspx