Ed,
It sounded like something that was related to "VISTA". Are you running Vista?
Also, you just said Nvidia card. If it is a 9000 series, there are some settings that caused issues. There is some discussion we can look up that has helped some.
I opened your file and it semed to work well. I tried to compute a simple planar toolpath and got an error that indicated something with the geometry I selected. I opened it in Rhino and had some surface oddities. I opened your BobCad file again and copied the large solid and padted it to a new document, then from there I saved it as an IGES and reloaded it back into BobCad and selected "Make one Solid" during the Import and Performed a Z level Rough with Innermost. (It also opened from there in Rhino as a nice Whole Solid).
I'll look at it a bit more to see if there is some surface duplications or something, and in the meantime, if your on Vista let me know and I'll post something for you to try. If Nvidia 9000, we'll have to dig up some possibilities