Windows Mail does not offer (do) anything relevant to your question. If
you want to archive messages outside of Windows Mail, then you can search
for previous answers to that question or ask it again. As Gary indicates,
disk space is cheap currently. You can get 1 TB drives for less than $100.
You can compress files and/or folders using Vista. To compress a folder,
go to the folder's properties, then click on the Advanced button, then
select the "Compress contents to save disk space" checkbox. You can do
something similar for individual files. If you have questions about that,
ask in a Windows or Windows Vista newsgroup or forum.
If your primary drive (C: or whatever) is limited in space but you have
another drive or partition with space, it might be possible to move all
your messages to a different drive or partition. I know that Outlook
Express made it amazingly easy to do that. If you want help with that,
then ask here about it but if might be better to create a new thread; I am
not sure whether a new thread or this thread would be best for asking
about moving the message store.