Skyhawk and Bill are correct
With the chip in the LVP configuration you only have a choice between loading an OS or re flashing the chip via LVP.
An experienced programmer could get around that with what SHOULD be a small change to the boot loader code.
Move the BLOAD switch to an unused pin: RB0 or RB1 should work. Tie the PGM pin to GND. Then use the bodged but working unit to program another 18F2550 with HVP. Install the new chip and move the switch back to RB5.
EDIT: Check original PK2 schematics to ensure the pin you use is actualy unused by the software!
and
AVI said the 18F2550 flashes the red light if he holds the boot button down as he inserts the USB. This indicates that the uC is running the bootloader and waiting for a OS download. I tryed keeping BLOAD pressed and XP sees the USB device and you can program it from the PICkit2 tool. Doing so will not result in a working programmer but it will tell you the USB is OK.