If you bought the products from a shop and it's written as OEM
on the CD, no sharing. It will automatically de-register you when
you try to register. No doubt you can activate it on both
computers, but registering will only work on one of the
computers.
If you bought the student's or teacher's edition, sharing is
allowed. Up to 3 or 5 comupters. I can't remember.
If you buy family edition, up to 5 is allowed.
