Hi
You need to have a USB driver/controller chip in your system such as the FTDI device, part number FT245BM or one of its variants. These are readily available, cheap and easy to interface to one of the AT89C51 8 bit parallel ports with a few of the other I/O line for chip control. The device comes complete with ready to use drivers, which work great and a set up routine so you can program your own ID string if you want to, these routines can be downloaded from the #FTDI website.