And to completely remove every trace of Plus!3 (_excluding_ the stored custom sound files which are in most cases located in a directory under the application directory of 
all user, and which can be used in Plus! Live too)
Check and if they exist, remove:
In registry key path:
HKCU\Software\Microsoft\Windows\CurrentVersion\Run
remove the key with name: 
MessengerPlus2
remove the key with name: 
MessengerPlus3
In registry key path:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce
remove the key with name: 
MessengerPlusUninstall
Remove the complete registry key path:
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\MsgPlus! Plugin
In the registry key path:
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows
if the key name: 
AppInit_DLLs contains the value: 
MsgPlusLoader.dll, then overwrite the value with an empty string.
Not sure about this one (even though this is for Plus!2 only):
In the registry key path:
HKCU\Software\Microsoft\Windows\CurrentVersion\RunServices
remove the key with name: MessengerPlus2
oh and don't forget to remove 
MsgPlusUninst.bat in your temporary files folder...
