Add A Message Box on Login

If your computer is used by all members of family, maybe you need set a message that will shown every computer booting up. You can put your own message in there, so you don't necessary to warn every user who will use your computer.

OK, to do it we need to modify registry. So you need open Registry Editor to modify your registry, just click Start - Run menu, type regedit in textbox then hit Enter to open Registry Editor.
In Registry Editor, drive your cursor to HKEY_LOCAL_MACHINE | SOFTWARE | Microsoft | Windows NT | CurrentVersion | WinLogon path. Then look for LegalNoticeCaption and LegalNoticeText keys. Modify LegalNoticeCaption will set the message title window and LegalNoticeText will set the message content. To modify them just double click on the keys. Begin from LegalNoticeCaption value first, double click it then type your desire title and then click OK button to confirm the modified value.
Then double click LegalNoticeCaption then type the message content and last click OK button to confirm it.
Close your Registry Editor and then restart your computer. When Windows XP screen appears you'll see your message box in there. And Windows XP will wait until you click OK button at message window before Windows XP continue the boot up progress.