The only thing I can come up with is the following.
If Messenger Plus! is installed on a limited account it cannot write its values to HKEY_Local_Machine in the registry so what it does is writes the values to HKEY_Current_User. Now SPNG checks the Local_Machine values to see if Plus! exists or not (atleast I think it does) But since the account I tested it on was limited and couldn't write to Local Machine SPNG wouldn't install.
I will let TB know of the bug when he comes online.
Or just post the bug in here:
http://shoutbox.menthix.net/showthread.php?tid=29201
But beyond Messenger Plus! thinking your account is limited thats the only thing I can think of.
/ 
