I'm running Windows XP SP3 and PBP 2.6 with MCS 3.0.0.5

The code is this:

DEFINE OSC 20

FLASH_RATE VAR BYTE
LED_0 VAR PORTB.0

FLASH_ON = 1500
FLASH_OFF = 500

lblLoop:
High LED_0
Pause FLASH_ON
Low LED_0
Pause FLASH_OFF

GoTo lblLoop

End

Again, this worked on my old PC with a previous version of PBP.