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Unfortunately Microchip no longer has the software on their web site, I have MPLAB 5.4 and 5.7 which I could email to you if you PM me. The files are 13MB or so each if your email provider allows this.
I'm not sure which software is exactly required to sure to run PICSTART, respectively what the latest MPLAB version supporting it. Said 5.4 and 5.7 won't run an modern Windows versions, you'll probably use a virtual machine.
These versions of MPLAB were designed to run on Windows 98, however on Windows XP you get read errors from the devices to be programmed which you can correct by deleting a running process with task manager, unfortunately I can't remember which process you had to stop. Does anyone know? I would be interested to know.