You can get rid of the button all together or just the flashing, see
my reply here.
As for your bug, I can not reproduce it here:
Windows 7 Ultimate SP1, 32 bits
Windows Live Messenger 2011 (15.4.3538.513)
Messenger Plus! 5.10.0.750
It is most likely due to you using an older Windows Live Messenger version there. Any reason why you are using an older version of Windows Live Messenger on the computer you get this on?