Flashing Hex File to AT89C51
The AT89C51 chip should contain an embedded USB boot loader pre-loaded on the chip.  The boot loader is used to program the flash.  The USB Boot-loader is described in this document.

http://www.atmel.com/Images/doc4287.pdf

The USB Boot-Loader works with the Atmel FLIP software.  The FLIP software is available for free from the Atmel website.

http://www.atmel.com/tools/FLIP.aspx