Macros can be digitally signed--it's a security thing to try to reduce the
incidence of malware (viruses, etc.). Do you perhaps have macros that you
wrote and signed using SelfCert from an earlier version of Word? If so, then
use the SelfCert tool that comes with Office XP (mine expires in 2008) to
create a new signature, and re-sign the macros with it.
Or... do you have an add-in that you obtained from a different source? If
so, contact the supplier and ask for one with a valid digital signature.
Or... are you unaware of any macros that are supposed to be running? If so,
then you might have a virus. Or, perhaps you installed something that added
something to Word without your knowing. The key here would be to remove
whatever add-in is causing the problem, unless it's a macro you know about
and want/need to use.
Does it say anything more about the expired signature?