Katrina
Do you mean accessing her hotmail or msn accounts?
If so there is no way to get them other than talking to them explaingin your concerns and ask to see the emails, Or failing that demanding your teen gives them to you or lose computer rights. I agree completely and wholeheartidly with Surreal2 comments regarding privacy and sensitivity, however remember that the terms for most internet messaging and email plans are with parents conscent for those who are considered minors. If you have not provided your teen with conscent, then you may choose to send an e-mail to the provider(s) and ask for suspension of the account.
If you dont want to confront the teen, then there are parental programs such as family key logger
http://www.spyarsenal.com/familykeylogger/ you can download it and it monitors key strokes on the computer and saves them in a log file that you can access by hidden keys strokes. You can view the file and esentially read what was typed on your end, but not the other computers so it is limited. Also you can get programs like actualspy
http://www.actualspy.com/screenshots.html that take snapshots every few seconds and log keys, giving you a greater level of control of what is happening on your computer.
I personally would not go to those extremes, but it is an option if you think you need it.