i think OP wants to use same email "profile" from both xp & vista. depends on the email app.
I'd try the email in operausb. http://www.opera-usb.com/
i run the same operausb browser from either xp or vista.
i think another possibility is to use any email app, but use an imap account. (but can do this only if have imap...)
more on opeausb: i also have two different "no-install installations" of operausb. both can run from both xp or vista. can run both operausbs simultaneously. to distinguish each, they have different color "skins". but I don't use the mail, so don't know if that may cause complications.