If you cut and past the registry entry into a file this can´t work.
Try
------snip-------
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
"Shell"="C:\\Program Files\\Team Mediaportal\\Mediaportal.exe"
------snip-------
I think you need the double backslashes.
Try
------snip-------
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon]
"Shell"="C:\\Program Files\\Team Mediaportal\\Mediaportal.exe"
------snip-------
I think you need the double backslashes.