In this example, we try to uninstall Citrix Virtual Delivery Agent using the Windows GUI.

1. Select the Windows Start Icon > Control Panel > Programs and Features.
2. Highlight Citrix Virtual Delivery Agent and select Uninstall.
3. In the XenApp pop-up box, click Remove.

The uninstaller fails, and message Removal Failed The following components failed to the removed is displayed.

We need to locate the Globally Unique Identifier (GUID) for the program.

1. In the Registry Editor, go to software/microsoft/windows/currentversion/uninstall
2. Highlight the uninstall folder and press Ctrl + F.
3. In the Find pop-up box, type the name of the program you need to install. In this example, we type citrix into the find pop-up box.
4. There will be many folders in a syntax such as {123456789-abcd-efgh-ijkl-123456789}. This strange string of text is the GUID. Make notes of the GUID for the program you wish to uninstall.
5. Notice in the example screen shot the UninstallString is MsiExec.exe /X{31FEF799-4364-407C-80A4-02D5FA3DBC10}.  In this example, in PowerShell type the command msiexec.exe /x{31FEF799-4364-407C-80A4-02D5FA3DBC10} REBOOT=ReallySuppress /qb!

This page has more information on msiexec.exe program: https://technet.microsoft.com/en-us/library/cc759262(v=ws.10).aspx