For information on installation order of multiple versions of Office, see http://support.microsoft.com/default.aspx/kb/928091.

Once multiple versions of Word have been installed on the same machine, you’ll notice that every time you switch between versions, it comes up with an irritating installer/configuration dialog before Word starts. To prevent this, the following commands can be run to add registry values that will fix the problem:

reg add HKCU\Software\Microsoft\Office\11.0\Word\Options /v NoReReg /t REG_DWORD /d 1

reg add HKCU\Software\Microsoft\Office\12.0\Word\Options /v NoReReg /t REG_DWORD /d 1

Thanks to David Overton, for this little tidbit.

David also writes about another problem where the EULA appears every time an application is opened. Read about it here.