"Now I'm off to figure out how to set up my 8051 with USB port as a USB device for windows to interface with."

You need to take the same approach as the Bluetooth module - just buy a chip or module that does all the USB "clever" stuff for you, and provides a simple interface to your 8051 and free Windows drivers.
Do not even think about trying to do this yourself!

See: http://www.8052.com/forum/read.phtml?id=47650

As you said yourself earlier,

"there is so much protocol and layer stuff going on that it's hard for me to sort it all out."

Both Bluetooth and USB are serious protocols; each one will take several man-years to develop from scratch.
Unless you specifically want to specialise in these protocol stacks, you should leave well alone!