This bug hasn't happened to me when using any 3.50 build, and no one I know has mentioned it, and no one on the forum has for that matter for that matter.
Please try the following
Uninstall both MSN Messenger and Plus
Start > Control Panel (classic view) > Add\Remove programs > MSN Messenger > Follow uninstaller...
Start > Control Panel (classic view) > Add\Remove programs > Messenger Plus > Follow uninstaller
Now restart your computer
------------------------------------------------------------------------
Now Download both
MSN Messenger 7 and
Messenger Plus and install both.
To download MSN Messenger 7
Click here
To download Messenger Plus
Click here.
------------------------------------------------------------------------
After doing this does the problem still occur?