Hello Mr Wolf,
On Msgplus.net's
Download Page you can find this information as well.
Messenger Plus! 5.0 is compatible with Windows Live Messenger 2009 and 2011 and can be installed on 32bit and 64bit versions of Windows XP to Windows 7.
Messenger Plus! 'Live' 4.9 is compatible with Windows Live Messenger 8.0 to 14.0 (2009) and can be installed on 32bit and 64bit versions of Windows XP to Windows 7.
So in other words, if you wish to use Messenger Plus! with Windows Live Messenger 2008, you'll have to download Messenger Plus! "Live" 4.9, which is also available for download at the
Download Page.
Hope this helps.
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