How secure is Hotmail, Gmail, etc.

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I recently install MSN Live (instant messager) on my new computer... and new means it never had MSN or had anyone logging into any of the msn site before. Strangely soon after I install MSN Live, it automatically logged me in as a user which isn't my personal hotmail account. That account has friends listed in her account and thus it seems to be an active account. Worst yet, when I click on the check mail icon, hotmail launched and shown me the emails for this person.

The question is, how safe is hotmail? Can someone else who install MSN Live can possibly see my account and all my emails without any authentication?

This can happen to anyone....