Why can't I install programs as an administrator?

Insure the regsitry keys under the following node have a value of "Enabled"

\HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\.NETFramework\Security\TrustManager\

References:

https://msdn.microsoft.com/en-us/library/cc176048(v=vs.90).aspx

https://msdn.microsoft.com/en-us/library/ee308453.aspx