Open your registry and find or create the key below.
Create a new DWORD value, or modify the existing value, called
"MessageExpiryDays" and set it according to the value data below.
Example:-
User Key:
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail]
Value Name: MessageExpiryDays
Data Type: REG_DWORD (DWORD Value)
Value Data: (0 = disabled, 3 = default)
Exit your registry, you may need to restart or log out of Windows for the
change to take effect.
Note you can save the registry key first by:
On the File menu, click Export.
In the File name box, type SaveKey, and then click Save.
Or create a system restore point prior to editing.
Note: If the registry key is not there, you can simply create one by going edit
http://support.microsoft.com/default.aspx?scid=kb;EN-US;322756
HOW TO: Back Up, Edit, and Restore the Registry in Windows XP
Disclaimer: Modifying the registry can cause serious problems that may require
you to reinstall your operating system. Use the information provided at your
own risk.