Normally some windows 8 drivers will also work on Windows 7 OS. Hence to begin with, you can try installing the Windows 8 driver and see if this works.
Also what is the model of the HP Scanjet that you have? Most of the scanjets now have a WIndows 8 driver released.
Since you already have a Windows 7 discs, I would recommend you installing Windows 8 from the recovery partition and then creating a partition on the HDD, and then installing Windows 7 for dual boot. Do backup your data before you perform a recovery else you will loose all the data.