Hi,
It could be a number of different things.
Personally I would start by reinstalling the printer driverand see if that works:
Unplug the pc>printer USB cable and then either uninstall by using the application provided (if there is one) or follow the generic print driver removal steps. Windows7=http://technet.microsoft.com/en-us/library/cc732946.aspx#BKMK_RemoveDrivers
XP=http://technet.microsoft.com/en-us/library/cc783972%28WS.10%29.aspx
Once uninstalled, reboot your machine and then carefully follow your printer installation instructions again (often you need to install the driver BEFORE reconnecting the printer).
Also, when you do reconnect the printer, try using the rear usb port first and only change to the front port if you get it all working on the back first.
Steve I, July 2010