Blank Screen reboot

Over the last four or five days my PC has started rebooting by itself fairly frequently. It only occurs when the USB broad band modem is plugged in, whether connected to the internet or not. The same modem works fine in my laptop which runs XP. I've run Virus checker, SpyBot and Adaware and can't find any bugs or viruses. Dr Watson is no help as close down happens so fast it can't write a log file.
Any ideas anyone?

 

problem solved

Modem is daisy chained through a usb splitter box.
Two of the ports have become non working, two are still ok.
Don't know whether this is hardware or software fault.
Will I attempt to repair or go out and get a new one!
I think the second track is the easier!!!!