Hi Jim,
If you buy a retail copy (ie with the installation DVD) you can leagally install, activate, and use Office Home and Student 2010 on up to three PCs/laptops in a single household.
If you buy the Office Home and Student 2010 Product Key Card, you can only use it on one PC/laptop.
See:
http://office.microsoft.com/en-au/pr..._Office_2010_5