Well, when you purchase Microsoft Office (which includes Word, Excel, PowerPoint, etc.), it usually comes with a serial code on the back of the box. The serial code is when enables the software to work, as Microsoft is very concerned with piracy of their programs. Depending on the version of Office that you have, you may or may not have more than one serial code.
For example, the Home/Education version of Office comes with three serial codes, meaning that up to three computers can use Word at the same time on the same network. But if you didn't have that version, or if your box only had one serial code, that means you'd have to buy another copy of Office to get another serial code.
Sucks, I know. But it's Microsoft's rules. So you need to get your hands on another serial code either by checking to see if you got a version of Office that has more than one code on the box, or buy purchasing another copy of Office altogether.