I'm not exactly sure why this could be happening, but it could be a corrupt install. I have heard of the CW 3D Graph causing this error before, and it was fixed by repairing the install.
To do so, launch the Add/Remove Programs Control Panel applet and select NI LabVIEW 6.1. On Windows NT/9x, click the Add/Remove button and then select Repair. On Windows XP/2000, click the Change button and then select Repair.
I am running Windows 2000 and it seems to work fine for me. I was able to place all 3 types of 3D Graphs to my front panel with no problem, so that's what leads me to believe it's a bad install.
J.R. Allen