Googling for this shows lots of people with similar problems (
http://www.google.com/custom?q=exceeded+usb+bandwidth+)
From having a quick read it seems like your gear is working as it should, i.e. sharing the bandwidth out between the devices.
You don't say if you have USB 1, 1.1 or 2.0 on the machines. Perhaps the difference between the laptop and the desktop is that the laptop supports USB 2 and hence provides more bandwidth? (USB 1.1 provides 12Mbits/second where as USB 2 is capable of 480Mbits/second.)
If so then perhaps adding a USB 2 card to your desktop PC would provide enough bandwidth to cope with the modem and the camera? They are about £15.
Alternatively drop the USB modem and get an ADSL router - £40 ish.