Downloaded Windows Live Messenger Beta, now can't find Windows Mai

I am kicking myself for downloading the Windows Live Messenger Beta as it
seems to have screwed up my whole system. I was happily using Windows Mail
and had my Yahoo mail sent through it (Premium Yahoo I think). Now I can't
even open Windows Mail. It's as though the program no longer exists in my
system. Nor can I go to the Yahoo home e-mail page. This is a nightmare.
That's where all my main e-mails go. Thanks for any suggestions.

P.S. In case it's important, my Windows Messenger is all connected to my
hotmail account which I rarely use. I just need to retrieve my Yahoo Mail!