Okay, I have downloaded the latest 3DCanvasPro, version 7.1.1.2 and now I am happy to report, it works.
The first time I tried to export the cube, it asked me a new question, "Include root group animation? If you are not sure, answer yes." Knowing better (there isn't any animation information...its a simple cube.), I took the advice, and it failed in the same way.
Next time, I answered no, and it worked fine. Then, I noticed that 3DCanvas Pro now exports to COLLADA. Well, now my money is twice as well spent.
No FBX, but COLLADA support is better than fixing the X export, imo.
I noticed that the latest XP updates have eliminated the remove buttons from almost all of my installed software in Control Panel, and of course you cannot uninstall .NET from that place, either. I did manage to remove the latest fix for IE7, which involved Outlook in some way. I don't ever use Outlook.
It also loads much faster now. I think that the latest 3DCanvasPro and the latest video drivers will go a long way in your case, too.