Likewise my USCC store says its another $25/mo. to tether. (What a bogus charge. They don't know whether the data flows from a tethered laptop or not.)
Anyhow, I use PDANet over USB and over Bluetooth. For USB, just make sure USB debugging is enabled on the phone and do NOT mount the phone as a drive.
For the Bluetooth tether, PDANet installs a DUN service on your phone. Go to Control Panel/Devices and Printers. Start PDANet on the phone and start the bluetooth tether. Say yes to making your phone visible and then initiate "find devices" on the PC. Do the pairing number thing on your phone. Now you're connected to the phone but not the 'net. Configure your PC to use a dial up connection, enter "123" as the ISP phone number, no username, no password and you are rocking. On reconnections, I've found the PC DUN will fail with a COM port error. Right click on your phone in Devices and Printers and deleted any COM ports left dangling. May also have to pair the two again.
I have not been able to the Bluetooth tether consistantly; always stumble a little, but it eventually works.