Windows 7 x64 doesn't ship with the driver for the 2600n, and won't recognize it if you try and add it.
If you try to get the driver for the 2600n from the HP website, you will find on run that it asks you to plug in the USB cable - which this printer does not have. Good one, HP.
However, for some reason, running the package up to the USB connection and hitting 'cancel' installs enough stuff for Win7 to correctly identify the 2600n.
So, the recipe is:
1. Download the latest driver from the HP website.
2. Run it.
3. When it asks you to plug in the USB cable, hit "Cancel" and quit.
4. Go to Control Panel / Devices and Printers and run Add Printer.
5. Select Network Printer, then IP address, and type in the correct IP for your printer.
6. W7 x64 should correctly recognize the printer at this point, where previously it did not.