Download and install the TweakUI Microsoft Windows XP PowerToy here:
...Follow that link and click to download TweakUI.exe
listed on the right.
Next, install and then run TweakUI. Should be in your start menu under Powertoys for Windows XP
Then, navigate to this view:
Uncheck the box Show unread mail on welcome screen
, and below, choose whether you want the changes to affect just you or all users on the system.
TweakUI will make the same changes in the registry as if you were to do it manually...only difference is it gives you a Graphical User Interface to make registry tweaking easier.