Cannot Uninstall/Reinstall OpManager

I have been using the free version of OpManager. For various reasons I have had to uninstall/reinstall it a couple of times. I have been trying to re-install it after de-installing it, but every time I run the installation, I get a message saying 'do you want to uninstall opmanager?' I say Yes, the process finishes, then I run the installer again to re-install, and get the same message again. I have checked the registry and the file system, and I can't find anything to do with OpManager in either, but as far as my system is concerned, OpManager is still installed.

I know this is the free version I am talking about, but if I can't get this working, my boss is going to waste 10-15000 dollars on Argent!

How can I manually uninstall and remaining traces of Opmanager?