Thank you for the replies they were all somewhat helpful eventhough none
really solves my problem:
1) Restoring the computer to a previous date it's what i'll be forced to do,
eventhough I don't like this kind of solution (i will have to reinstall more
recent programs and i always fear to lose data).
2) I tried repairing the Word installation once, it didn't work, then i
uninstalled it once and reinstalled it without positive results. Finally i
uninstalled and reinstalled the whole Office package, but it didn't solve the
problem.
3) I thought about using OpenOffice, but the compatibility sometimes is not
just perfect with the latest docs.

Thanks to everyone.