I had that problem last year when I tried to tether to my Dell D620. After a lot of Google searching, I discovered I had to replace the Toshiba bluetooth drivers with the Microsoft Windows native bluetooth drivers. I saw "bluetooth stack" mentioned a lot. Start with this Dell KB article:
Finally got rid of Toshiba Stack in my Dell and replaced with Microsoft stack. There must be an easier way than what I did. I'll have to recreate and document in simple English.
