"It might be easier to use one of the Cypress '51 chips with built-in USB."

There is absolutely no question whatsoever of "might" here. To develop a USB protocol stack from scratch with no prior experience is a huge undertaking - we're talking man-years here without a doubt!
And some more man-years for the Windows drivers!

You definitely want to go with one of the processors with on-chip USB, or use an addon USB chip or module. Most also give you free Windows drivers.

Search this forum for "USB" and you will soon find some recommendations; eg, here's mine:
http://www.8052.com/forum/read.phtml?id=41749

Take a look at the USB section on the Keil site:
http://www.keil.com/usb/

Here's a Triscend App Note:
AN19: An E5 Full-Speed USB Reference Design Using the FTDI FT8U245AM USB 1.x Controller