Hi,
In my opnion Yes, you can. However you may want to take care of few things
such as
1. XP and Vista are installed on different partition of the hard disk
(differnet drive, say C and D)
2. Also you need to re-install office on both the operating systs. You can
specify the same drive for the office installtion. I mean if XP is on C drive
then install Office on C:\Prog File and Vista is on D drive then install
office on D:\Program files
HTH,