How to force the uninstall of an MSI

I got the following message :

Another version of this product is already installed

After a while, I finally found this post :

• Execute :

msiexec /i program_name.msi /lv logfile.log

• Find the GUID (look for Product Code from property table before transforms: ‘{GUID}’)
• Zap it

msizap.exe TWP {GUID}

I found msizap here