Well, yes you do need a valid key (i.e. the Product key that you get after buying the product) if you want to access Microsoft Office 2003, however currently you can get the beta version of Microsoft Office 2007
(until February I think... after which that too will expire). But the only problem is that it is not as reliable as the other version and might crash a couple of times... which might get very annoying.
And OpenOffice actually is just as good, infact its better since its free. It opens all kinds of files... even .doc, .ppt etc but just that it might lack a few features since it is open source.