Probably has to do with security rules in NTFS: not having write access to other users accounts. Might work if you're an administrator. It's not an Office issue, it's a feature of the filesystem.
I find it easiest to make new folders for such shared files, outside everybodies Documents or place them in the Shared Documents.
I have docs and excel sheets I created in Word97 & Excel97 on a Windows XP sp2 Dell computer. I can copy and use them in my HP Windows Vista computer using office 2003. I can make changes to it, but when I try to save it, it brings up the SAVE AS dialog box (instead of a direct save) and says it's read only and won't save my changes. I looked under properties and the read-only box is not checked.
OK, more info??It seems to be because I put these files under a different user name than the user account I am in. So if I copy the file to the current user's documents then I can save changes to it. Oh, I CAN save it to the current user's documents.
So I quess the real question is why can't I save a file (in Vista) under another user name?