You will have to buy the full version unless you have a qualifying product.
The list of qualifying products for Office 2003 than it was for Office XP -
includes Works for example. You can find the qualifying product list by
select your Office edition here:
If you hve a beta copy on the system and it is not the final RTM copy then
you will need to format as well. There have been issues in having the beta
on the system and trying to uninstall the beta and then install the retail.
Doing the standard removal and even the mannual rip using the registry
removal and program folders did not totaly fix any issues.