Do you have any programs running that prevent writing in the registry (like AdWatch, part of AdAware)? Id so, turn that off and reinstall Plus! once more.
Is C:\Program Files\MessengerPlus! 3 still there? If so you can restart MsgPlus! through taskmanager (ctrl alt del - 1x - file > new task..browse to Msgplus.exe). After you've done that i suggest you set the hookingmethod of Plus! to
Alternate mode (click link for info on how to set it properly). Incase there is any program interfering with Plus! this is the prefered hooking method.
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