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_MACHINESoftwarePoliciesMicrosoftWindowsInstaller and changed the ‘DisablePatch’ entry to 0.
Bingo! This time the installation was successful.