This error is mostly caused by a corrupt MSXML DLL (You don't have to know what it is or what it does). According to the Windows Live Solution Centre, it can be fixed by following
these instructions
In the case that it doesn't, they recommend doing a
"Clean Installation of Messenger"
I know there are a lot of instructions to the second link, but it will be the best option if the first doesn't fix it. You
must follow
all instructions exactly. If you don't neither method may work or you have missed something that we will assume you have done and could lead to confusion if we have to further troubleshoot your problem
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