You need to use the IAP (In Application Programming - "self-programming") functionality of the DS89C450 - please read the datasheet. It has nothing to do with the PC nor its OS.

However, the DS89C450 is one of the least suitable '51 derivative for storing data in its internal FLASH, as the FLASH has to be erased in too big chunks (half of the whole flash if I recall right) before reprogramming (or sophisticated algorithms have to be employed to reduce the need forcerasing each time, storing at different places of FLASH and marking already invalidated parts etc...). Depending on the particular application you might better ise an external EEPROM or alike; or use a different derivative with FLASH erased in smaller blocks.

Jan Waclawek