Unable to Install Silverlight

I was unable to install Silverlight on a Windows 2003 server, it failed with error 1625.

I took a look in the %TEMP%/SilverlightMSI.log and found this entry:

This installation is forbidden by system policy. Contact your system administrator.

This was strange as we don't have any GPOs to restrict software and UAC wasn't a factor.

A few lines up there was another entry:

Machine policy value 'DisablePatch' is 1.

I opened up Regedit, navigated to HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\WindowsInstaller and changed the 'DisablePatch' value to 0.

Bingo! This time the installation was successful.