Posting this for a friend.
After reformatting and reinstalling windows etc, he installed his modem drivers as normal... but when it came to actually connecting the modem to the PC, the computer crashed on him (and does this every time he has retried).

Note that the modem has worked fine on his computer in the past, he has no spy/ad/malware/viruses etc on the computer; a crash is only triggered by connecting the modem (other ports have been tried, same problem) and other USB devices work ok.

When he phoned his ISP for advice, he was told it could be an IRQ conflict, however, no conflicts were found after he checked device manger.

The modem manufacturer is Sagem, don't know the model. Apologies if this has been solved already or if I haven't explained well enough.

I had the same problem. it was sorted by putting in a new power supply. If the one you have in is failing and not putting out enough wattage the power drawn by the usb modem makes the motherboard crash because of lack of power. you can check by plugging in any usb item that requires a resonable amount of wattage i.e anything which charges through the usb and the computer should crash if its the power supply