You cannot plug your scanner and existing cable into a LPT port and expect it to work. That is in fact a SCSI cable you have, not an adapter cable. You need a SCSI card with a DB-25 external port such as an Adaptec AVA-2906.
http://www.adaptec.com/worldwide/sup...imageWidth=600
I'm sure you could find a cheaper offbrand card somewhere, I picked the Adaptec only as an example.