I would suggest opening a helpdesk ticket. But here are a few ideas:
It sounds like this assembly in the gac is corrupted. If you have another intstall of the same software version, you could try copying the assembly from that one and installing it in the gac on this install. If not, when you uninstall, make sure this assembly is gone from the gac. You might also want to download a fresh copy of the install files to make sure they are not bad.
Although I have not seen it myself, I have heard of cases where something goes wrong with the whole gac. In that case, you might try un-installing and re-installing the .NET framework.
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