Is there a way (GPO preferred) to disable the "Take a tour of Windows XP" balloon/popup when a new user logs in for the first time?
The registry key's value is REG_DWORD type, setting it to "0" will disable the pop-up.
You can set this value in GPO using a logon script or, preferably, Client-Side Preferences. A logon script would look something like:
Edit: Here's an ADM that should perform the same function, if you're into that kinda thing.
Per @natxo's comment below, the above will disable the pop-up on a per-user basis. You can use this to target specific user groups/OUs if you want. Should you wish to disable the pop-up on a per-computer (all users) basis, change "HKCU" to "HKLM" in the script or change "USER" to "MACHINE" in the ADM. This is detailed in the Microsoft Support link above.