Word docs created on Windows98 can work on Word versions on 2000/XP?

My organisation is replacing “old” machines (Windows98) by newer ones, that come with XP or 2000?
Word documents created on XP – that use merging – work well with 2000, but not with 98. Also, documents created on 98 cannot be opened in XP or 2000. It seems alignment is not working well either…
One solution, not desired, would be having two diferent files, identical but for diferent versions: one for 98, the other for 2000/XP.
Is there a way to configure Windows or Word, so that a file can be opened in any of the mentioned Windows/Word versions?