If it's the Student and Teacher Edition, you are allowed to install Microsoft Office for Mac on up to 3 PCs in one household.
If it's the other edition, you're only supposed to install it on one PC. Can it work on more than one PC? I'm not sure though I think it can. The only way to find out is for you to try (I'm not condoning this).
OpenOffice and NeoOffice are both available for free. You can use the free Office suites until you scrape up enough coins to buy another license, if need be.