WiX Burn - How to set a checkbox if a registry key does NOT exist

Currently my WiX bundle is doing the exact opposite to what I want it to do ?!

I have a Checkbox defined in my theme:

<Checkbox Name="cbInstallTraining" X="11" Y="-85" Width="222" Height="17" TabStop="yes" FontId="3" HideWhenDisabled="yes">Install/Update Training Material</Checkbox>

and my variable defined as:

<Variable Name="cbInstallTraining" />

And what I'm trying to do, is search the registry to see if one of two registry keys exist and if neither exits, then I want to set the checkbox to be ticked. If either of the two registry keys DOES exist, then I want the checkbox to be un-ticked.

So currently the opposite is happening, if a reg key is found it's setting my cbInstallTraining variable and hence the checkbox has a tick in it.

Hope that makes sense and that someone could enlighten me on how to get my desired result ?!

Cheers,

Chris.