Can you remember whether you bought an OEM version or a retail version?
If you bought the OEM version you can only install it on one computer. If you bought the retail version you can install it on up to three computers.
If you want to be absolutely certain, download and run Belarc Advisor, which will take an inventory of your computer including the product keys for any software you have installed. http://www.belarc.com/free_download.html
If you see a product key for Office 2007 in this format - xxxxx-OEM-xxxxxxx-xxxxx - then you know it is OEM software that can only be installed on one computer.