Try this first.....power down your printer. Then, disconnect the USB cable connecting it to the computer. Then, power the printer back on. Once fully powered on, reconnect the USB cable. This may force Win 7 to redetect the device and install the drivers you already have installed (I assume it worked at some point).
If not, go to this link and get the latest drivers. Installing these should remove your old drivers and replace with the newer drivers:http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?product=79499&lc=en&cc=us&dlc=en&lang=en&cc=us