My Canon MP 390 USB Printer hangs XP boot on a Dell 8300.
Boots fine with printer not hooked-up. Boots with other USB devices. All USB hubs show up in Device Manager. Clean re-load of XP, SP2 and current BIOS don't help.

Maybe an IRQ conflict? Or What?

USB doesn't deal with IRQs so that wouldn't be the issue. I would try a different cable, but its possible the Canon has gone south on you.

Does the printeer work if you turn it on after the PC has booted? Or does it crash the machine?

If it works it may be worth looking at your system bios for startup options relating to usb.

If it doesn't I think one of your drivers, cable, device is buggered. Easiest one to check is remove drivers and download new ones!