It is attached via an Adaptec 2940 card. The Windows 7 driver has to be installed manually, use the AIC-7870 PCI SCSI (Emulated) driver. Once installed the scanner is immediately identified as an 'HP 2520A SCSI Processor Device' and listed under 'other devices'.
Now you need a driver. Go to HP and download the sj166en.exe executable WinZip package. Find the setup.exe application in the DSIK1 folder and run as admin. Leave the SCSI adapter choice as the default. When it's done you'll need to restart.
Now everything works find and results look good. The interface is a bit clunky but what do you expect for such old software?