Any errors besides "MMC cannot open the file c:\windows\system32\dsa.msc"? Have you attempted this action from a remote computer, and did it work? If it worked, I would say that there may have been a corruption in the .dll's, or you don't have permissions to either the .msc, or the MMC on the server you are attempting to open from.
On the DC, can you open up other MMC's? e.g. Sites and Services, Domains and Trusts?
If you can then all you need to do is copy the dsa.msc file from another DC (I am assuming you have at least 2 DCs) and replace the dsa.msc on the troublesome DC.
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