Imagine a situation where you want to pop-up a legal notice and the users read this legal notice before logging when a user starts on a particular PC. You can simply accomplish this by some simple registry editing.
Start] [Run] [Regedit]
Go to: HKEY_LOCAL_MACHINESOFTWAREM*cf*tWindows NTCurrentVersionWinlogon
Modify/Create String Value of Data type REG_SZ Named [LegalNoticeCaption]
Value Data for LegalNoticeCaption: [Type the Caption for the Dialog Box]
Modify/Create String Value of Data type REG_SZ Named [LegalNoticeText]
Value Data for LegalNoticeText: [Type the Data for the Legal Notice]
Exit Registry / Reboot.