When installing the ShoreTel PCM 9.1 call manager software on a PC that has been upgraded to Windows XP service pack 3, occasionally there is a Microsoft Installer error that prevents the ShoreTel software from being installed. The error is “Error 1720 – A script required for this install to complete could not be run”. The installation will then fail requiring the issue to be corrected and the installer to be ran again. This is caused by an issue with the Window XP service pack 3 install that sometimes will corrupt certain binaries used by the OS. Microsoft recommends a reinstall of the “Microsoft Distributed Transaction Coordinator” to correct the issue. This challenge is usually only noticed in a handful of service pack 3 machines where the corruption is present. http://support.microsoft.com/kb/891801