The drivers for Windows 7 have become obsolete hence why you have to use the In-OS drivers. So I found this document that covers just about every possible step to resolve this for you.
If this doesn't help, is your Windows 7 PC 32-bit or 64-bit?
First step is to be sure the USB printer cord works. Then when I selected to Add a Printer from the Devices and Printers folder, I choose the USB001 option (not LPT) then selected Windows Update (which took a long time to load). On the left I selected HP and on the right I found the Deskjet 3600 series but you may also be able to use the Deskjet 990c drivers as well if the 3600 series doesn't load.
I really hope this helps!