Where is the registry key that point to location of $NTUninstall folders?

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http://serverfault.com – I am running out of room on the C: drive on my Windows Server 2003 R2 Server and want to move the $NTUninstall files to D:\Uninstall_Folder and update the registry key that tells Windows where to look for them. I have seen this done in the past but can't seem to find that registry key. (HowTos)