"Reference-Style" Answer: This is an alternative answer to the one below with several different options shown. Uninstalling an MSI file from the command line without using msiexec.
The command you specify is correct: msiexec /x {A4BFF20C-A21E-4720-88E5-79D5A5AEB2E8}
If you get "This action is only valid for products that are currently installed" you have used an unrecognized product or package code, and you must find the right one. Often this can be caused by using an erroneous package code instead of a product code to uninstall - a package code changes with every rebuild of an MSI file, and is the only guid you see when you view an msi file's property page. It should work for uninstall, provided you use the right one. No room for error. If you want to find the product code instead, you need to open the MSI. The product code is found in the Property table.
UPDATE, Jan 2018:
With all the registry redirects going on, I am not sure the below registry-based approach is a viable option anymore. I haven't checked properly because I now rely on the following approach using PowerShell: How can I find the product GUID of an installed MSI setup?
Also check this reference-style answer describing different ways to uninstall an MSI package and ways to determine what product version you have installed:
Uninstalling an MSI file from the command line without using msiexec
Legacy, registry option:
You can also find the product code by perusing the registry from this base key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall . Press F3 and search for your product name. (If it's a 32-bit installer on a 64-bit machine, it might be under HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall instead).
Legacy, PowerShell option: (largely similar to the new, linked answer above)
Finally, you can find the product code by using PowerShell:
get-wmiobject Win32_Product | Format-Table IdentifyingNumber, Name
Similar post: WiX - Doing a major upgrade on a multi instance install (screenshot of how to find the product code in the MSI).