This is what I should do :

1. disable the serial interupt (bit ES in IEN0)
2. Retry the procedure.
Does it crashing again ?

An oscilloscope can do miracles. Look what is happening on the RX pin 10 of the 8052 when you start your PC.
Offcourse the PC send some data when it is booting. Windows always check for plug and play components. (if you're using windows ?)

Check also you're software in the 8052...
Does it reset the RI bit (bit 98 in SCON) when the buffer is read ?. If not, the 8052 will re-enter the UART routine after the RETI instruction. An other posibility is that the STACK in the 8052 goes into a overflow state.
Check : disable REN (serial reception) on entrance of the UART interupt routine, and enable it again before RETI.
Filip