I have the same problem but delete the thumb files doesn't solve the problem in my case...
Strangely I've tried the programs (TreeMagik G3, G2 and Plantlife) on Windows Vista (on the same PC) and all works well...
But i need to use them on XP Pro.
I'm a bit disappointed since trying the demo versions I was having the same problem and contacting the presale support they say me that was a problem only in the demo versions and the full versions weren't affect by the same problem...
I've waited for a while and finally bought the programs but now found that the problem is the same.
I hope they will have a solution for the problem or I definitely want back my money, I don't wanna waste 100 $ !
Dzeta.
(to be or not to be ...serious LOL!)