You can download and install *TweakUI for XP*
Powertoys for Win XP Pro
http://www.microsoft.com/windowsxp/pro/downloads/powertoys.asp

Powertoys for Win XP Home
http://www.microsoft.com/windowsxp/home/downloads/powertoys.asp

To disable unread mail notice for all users
HKEY LOCAL MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\UnreadMail
MessageExpiryDays   change to    dword:00000000

To disable unread mail notice for current user
HKEY CURRENT USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail
MessageExpiryDays   change to   dword:00000000

To simply clear the current unread count.
HKEY CURRENT USER\Software\Microsoft\Windows\CurrentVersion\UnreadMail\

<your email account>MessageCount   change to   dword:00000000

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 > new.

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.