I'm a Windows 2k Professional, Build 2195 Service Pack 4, user
To solve the problem with the 1603 error during the installation of the MSN Messenger 6 I did the following:
1. Decompressed the installator file, using WinRAR, in any folder
2. Ran the file MsnMsgs.msi and a dialog box appeared saying that there was a problem creatin' a folder inside \Documents and Settings\Administrador\Configuración Local\ (my Windows is in spanish) so I cancelled the installation
3. Verified the Security Directives of the folder (right click, Properties, Security tab) and realized that the system didn't have any permissions so I created them (Add button next to the users list, pick SYSTEM from the list, Add it and OK).
4. Ran the MsnMsgs.msi again and worked out well!
I hope I could've help somebody, sorry if the explanation it's for dummies, I ain't very good with these things.
Novalis