Click Start.
In the Search field, type regedit and hit Enter.
Locate the following registry folder: HKEY_LOCAL_MACHINE\ Software\Microsoft\Windows\ Current Version\Policies\ System.
On the right pane, double click the legalnoticecaption and select Modify from the menu.
In the Value Data field, type whatever text you want. (this will be the Title of your message)
Click OK.
Now, double-click the legalnoticetext registry key and select Modify from the menu.
In the Value Data field, type whatever text you want. (this will be the Body of your message)
Click OK.
Now, restart the computer for the change to take effect.
When your system will startup you will notice your custom message!