USB program loading

I have a couple of projects that use PIC18F4550 chips, and i want to set them up so i can load new programs via usb. i write my programs in swordfish, which gives me the hex files.

What is the simplest way i can set the chip up to program over usb, (not hardware wise, just the software i need on the computer and on the chip)

I have tried following the stuff i found online but have never been able to compile them and load them on the chip or computer.

thanks

Check out this site, which does just what you want:

UBW (USB Bit Whacker) Project