OK... good news.
Opera works with Vista and Outlook webmail. It doesn't display everything correctly, but i was able to access all my folders, inbox, and create new messages without an issue.
As for internet explorer, there seems to be a work around. When you log into webmail you usually have the option of "premium or basic." If you choose basic you'll be able to send and reply to messages. But you lose some of the nicer features of outlook webmail. But again, u can still access everything.
Hopefully your company's version of outlook webmail has those two options. If it doesn't, download and use Opera.