It turns out that Windows Vista has a bug that prevents you from opening and saving a compound document file back to the same name unless you're logged in as an administrator on the machine. There's a hotfix available, which is good. The fix isn't in Vista SP1, which isn't.
Obviously no one at Microsoft tested this on anything earlier than the 2007 versions of MS Office, since all the earlier versions use compound document files for storage. Or else they never logged in without administrator rights.