Hello I am an employee at HP and I just replied to another person on the same type of problem. I don't know if you already did this but I had the same problem on a different printer and it worked for me. You have to have the printer turned on and hooked up to your PC and then do a Windows Update.
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With Win7 update the process will see that you have the printer and need a driver and should give you the option to download it. You can then select it and download it and it should work. Here is also the text from the HP website.
I hope this works for you.