Your program writes to the pins used for ISP. This may be locking the PC, or preventing the 8252 from entering ISP mode. Try keeping the 8252 in reset during power-up, so that the program does not get a chance to run.
And if you insist on writing wild things into the ISP pins, add a line at the beginning of your program that give you the ability to "opt-out" of running your program...
LETSGO SBIT P0^0
...
main:
JNB LETSGO,$ ; if LETSGO low, stay here to help ISP
; go wild