either the port, the driver or the printer. The test page is done locally inside the printer and that doesn't say anything about the electronics of the interface with the PC.

- I'd start with powering off everything (PC + printer), reboot and try again. Maybe you've done so already, but you don't tell about it.
- Then check the same USB-port with any other USB-device (mouse, USB-stick, mp3-player, whatever you have).
- Then try the printer on all of the other USB-ports.
- Then delete the printer from Start>Printers and install it again (according to the installation instruction)
- Then install the printer (according to the same installation instruction) on another PC.

Somewhere in the results might be the clue. I assume there's nothing specific like "immediately after I installed SP3" or "after I dropped the printer" or "after I removed 100 viruses" that could be a cause.