have you tried this driver, and followed the instructions? i got an officejet v40, and had that problem, i had to get driver from site, and reinstalled it like 4 times, till i finally followed the instructions and it worked.
AFter i installed the hp psc 750 the add new hardware wizard pop up saying hp psc 750. The new hardware wizard attempoted to look a driver for the scanner but to no avail.