Welcome to the forums Ian and Martin.
If it is the USB light going out and not the cable/signal light, It would appear to be a usb problem.
I know its a pain, but you could try checking that both ends of the cable are securely inserted and when they are in, see if there is any play (wiggle) in the socket. I had my cable modem on usb and it would disconnect when accidentally kicked the PC desk. (happened alot when dying on counter strike
)
If you have a usb printer, you could also try connecting the modem with that cable instead to see if it helps, also as florence suggested try using other usb sockets on your pc.
On a side note, are you sure that your pc dosnt have an onboard ethernet port? Alot of motherbaords these days come with them, and generally they are far more reliable than USB for the Cable modem.
Good Luck