Before a client can use your custom interface, the proxy/stub DLL for the interface must be installed in the system registry. Registering the proxy/stub DLL involves creating a .REG file and running REGINI. In the following entries, notice that the IID for ICustomInterface is the same as the CLSID for the proxy/stub DLL.
\Registry\MACHINE\SOFTWARE\Classes\Interface\
{7ACC12C3-C4BB-101A-BB6E-0000C09A6549}
= ICustomInterface
\Registry\MACHINE\SOFTWARE\Classes\Interface\
{7ACC12C3-C4BB-101A-BB6E-0000C09A6549}\ProxyStubClsid32
= {7ACC12C3-C4BB-101A-BB6E-0000C09A6549}
\Registry\MACHINE\SOFTWARE\Classes\CLSID\
{7ACC12C3-C4BB-101A-BB6E-0000C09A6549}
= ICustomInterface_PSFactory
\Registry\MACHINE\SOFTWARE\Classes\CLSID\
{7ACC12C3-C4BB-101A-BB6E-0000C09A6549}\InprocServer32
= proxstub.dll