I have tried, but been unable, to get "Internet Connection Sharing" to work over a Bluetooth connection.
The problem is that, in order to enable Internet Connection Sharing, there has to be a free network adapter to connect to the client PC. Windows doesn't recognise the Bluetooth adapter as a valid network adapter for this purpose. I've managed to get file sharing to work over the PAN (Personal Area Network), but that's it. With a file transfer speed of 1Mbps, it is not exactly a fast network!
Why though are you trying to share the desktop's internet connection instead of connecting the laptop directly to the router? That, after all, is the purpose of a router.