In most cases, one license entitles you to install Microsoft Office on only one computer. However, depending on your license and software version, you may be able to install Office on a portable computer or up to three machines your family uses.
Your Software Rights
The End User Licensing Agreement found on the shrink wrap or installation screens of a piece of software explains your rights to use the product. In general, software is licensed only to a single computer, though many users may use that computer. Microsoft does not permit users to "split" a license between multiple desktops by only installing certain programs in the Office suite. If you're selling or giving away an older computer, you can reassign an Office license to your new computer, though you must completely uninstall the software from the older machine.
Secondary Portable Installation
Most retail versions of Office allow a single user to install a second copy on a laptop, tablet computer or other portable machine. The second installation is intended to be used by the primary user while travelling, at home or at a satellite office. The terms of the license make it clear that only the primary user can run the software, and Office itself enforces that requirement. If a second user opens an Office product on your portable machine, it connects to the Internet to check if another licensed copy is running. If it detects the license code is in use, the application will politely inform the second user and quit. Legitimate use on two machines simultaneously (comparing two versions of a spreadsheet, for example) isn't prohibited by the license, though you will need to turn off network connections on the second computer.
Home and Student Installations
Retail purchasers of Office Home and Student 2007 are entitled to install Office on up to three home computers with a single license. Despite what the name implies, you don't need a student in your household to qualify for installation on multiple machines. However, the license only covers noncommercial use; if you work from your home, you'll need to buy the appropriate version of Office. Users must live in your household, so a college student living away from home cannot take advantage of the license. An exception is a vacation home used primarily by your family. The terms of the licensing agreement consider a vacation or summer home to be an extension of the primary residence.