Shoutbox

This problem seems to have the 'symptoms' of what happens when Plus! is installed on the newer Windows Live Messenger (I understand you have said above you are not using this).

Have you, in the past, installed any version of Windows Live Messenger on that system?

Also as a test to see if all of these 'symptoms' are the same as what has been occuring with Plus! and WLM, try and set transparancy to your contact list. Next, right click on a contact in the contact list and see if there is a Messenger Plus! menu at the bottom.

If you are able to set transperency but are unable to use features via right clicking in the contact list then your problem pretty much looks like Plus! installed on a Windows Live Messenger system, even if this is not the actual case.