Helping people with computers... one answer at a time.
Depending on the situation, the number of machines you can install a single copy of Microsoft Office on is unclear.
I wanted to know if we're allowed to use a legal copy of Microsoft Word (legally meaning I purchased the disk) on more than one computer. I switched computers and now I need to install it on a different computer than the one I'd been using before. The old computer is still active. Can I re-install on my new computer? Will it recognize and allow me to use the product key?
If you're moving from one computer to another, the answer is easy. But if you're adding an installation, the answer isn't quite as clear.
If you're moving your installation of Office (or Windows, or pretty much any licensed software package) from one machine to another, the answer is simply "yes". That means you plan to stop using the package on the old machine, and start using it on the new. No problems, no conflicts, and no questions. About the worst this scenario might get is with over-aggressive anti piracy techniques that might require you to contact the software's manufacturer to verify that you're moving, and not copying, the installation in order for its activation to succeed.
On the surface, copying is also simple: it's illegal. Regardless of whether the application can be installed on another machine, and whether or not it works, most software license are "single seat"; meaning that you're allowed to have the software installed on only one machine at a time.
In practice things get just a little grayer than that, though. Some software publishers have license agreements that state you may install on a certain number of machines as long as only one is in use at any time. Others allow you to make a single copy of the software as a backup.
For Microsoft Office specifically, I was lead to believe some time ago that one could install it on up to three machines for personal use. Today I must assume that's wrong, as I can find no documentation to back that up.
There are plenty of options besides purchasing an additional copy of Office, especially if you can't justify the additional cost. Consider: