Probably because of the number of drivers and hardware layers involved...
Application
Windows serial driver (probably not, it should just skip this driver...)
Xircom's serial<>USB gateway
Windows USB driver

Intel Chipset
USB interface
Xircom USB chip

Xircom USB chip's firmware
Picstart+ / ICD

"I've finally learned what `upward compatible' means. It means we get to keep
all our old mistakes." -- Dennie van Tassel

-Adam

Mike Harrison wrote:
>
> For your info, MPLAB 5.00, Picstart + and ICD appear to work OK on an
> Entrega (now Xircom) USB-to-Serial interface.
>
> At first MPLAB greyed-out the USB com port's number, but manually
> setting the port number in MPLAB.INI got round this.
> Picstart gives occasional 'cannot receive' errors but seems to be
> pretty useable. ICD appears to work fine.
>
> ..Why am I surprised that this worked....????