It doesn't launch Messenger if Messenger Plus! must be loaded (in fact it appears and disappears in miliseconds) but when I renamed msimg32.dll to another thing, it is launched successfully. Also if I am launching a second Messenger client from toolbar's button, it is also launched successfully. The problem occurs only for the first client.
Windows Live Toolbar's Messenger button version 03.01.0000.0146 (I couldn't find a unique version number for toolbar itself
)
Windows Live Messenger 8.5.1302.1018
Messenger Plus! Live 4.50.311
Windows XP SP2
(no need to write the skin I use, because the problem occurs even in a non-skinned Messenger)