I think you have to go to add remove click on Windows live Essentials and at the top it should give you the options of a repair.
Now this is where I am not sure ,You can either choose Windows Live messenger out of a list or you have to repair all of Windows Live essentials.
Yes just tried it it gives you the option of just repairing the one program and not all of them.
So obviously you pick Windows Live messenger.
Worth a try if nothing else works.