The only way I can see that working is if you boot from the external drive when you need to access Microsoft Office. Bear in mind that this is only a guess from me as I have never tried it. The OS used will be the one on the drive you boot from.
Okay,thanks for your response. I guess whole 'switching OS's' thing might be too complicated for what its worth. So maybe you can help by answering this: Will I be able to use MS Office from external drive, or is it somehow working through that particular drive's OS?