Make sure you backup the registry before deleting any Keys and export any keys in the registry before you delete them so that they can be added back in if necessary.
Search HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products for 'Touchpaper'. You should find a match in the InstallProperties key under a KEY which is a GUID. The One you are interested in has a String Value named 'DisplayName' which contains 'Helpdesk'
You should find a match in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products\AE569B9956CC277479872599C08BF90D\InstallProperties.
You will need to delete AE569B9956CC277479872599C08BF90D and everything below it (The Key on your machine may differ from the one in this example).
If you are having issues installing Helpdesk Internet edition then you should find another key with a display name of 'Helpdesk Internet Edition' which will also need to be removed.
You will also need to search HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall for 'Touchpaper' and remove any Keys you find.
You will need to delete:
(The key names may differ from the ones in the examples above).
Once this has been done. Reboot the PC and try installing Helpdesk.