This is done with this DWORD in the registry:

For Outlook Express 4.0 :

[HKEY_CURRENT_USER \Software \Microsoft \Outlook Express]
NoSplash=1

For Outlook Express 5.0 / 6.0 :

[HKEY_CURRENT_USER \Identities \{GUID} \Software \Microsoft \Outlook Express \5.0]
NoSplash=1

For Windows Mail (Vista) :

[HKEY_CURRENT_USER \Software \Microsoft \Windows Mail]
NoSplash=1