LucyLou, this forum is for Windows Live OneCare - http://onecare.live.com - so, you are off topic here.
Are you using Windows Live Hotmail on the web at www.hotmail.com and your older messages disappeared? If they were in your Deleted items folder, they will be cleaned out periodically. They should not be removed from any other folder unless you have not logged on for a period of time (I believe it is 30 days) and then all messages are removed.
If it is a Hotmail account, and mail is disappearing randomly, contact support - http://support.live.com
-steve