This topic has been brought up from time to time on this site. A search would yield to plenty of information.

In summary, you can have following methods.

1) Choose a derivative with USB support.

2) Make your 8051 board with RS-232 port and connect it to PC using some ready made USB-RS232 converters

3) Use some USB-232 converter chip in your design. I rememebr one such chip is from FTDI, very simple to interface

http://www.ftdichip.com/FTProducts.htm