First download HPLIP current version for Slackware hplip-1.6.9-i486-1 from here
Download to /home/username
Now to get this to work hpijs has to be removed to do this open a terminal and su to root
at the prompt Type
# removepkg hpijs
Once this is complete and you have the prompt type
# installpkg /home/username/hplip-1.6.9-i486-1.tgz
Next you have to start hplip before cups to do this at the prompt type
# /etc/rc.d/rc.cups stop
# /etc/rc.d/rc.hplip restart
# /etc/rc.d/rc.cups start
Install your printer using localhost:631 in your web browser or if you use Kde you can use the printer install wizard, you will need to use the PSC 1600 foomatic/hpijs driver (part of the HPLIP package and reinstalled with that package, if the hpijs drivers are not removed prior to install of HPLIP the scanner will not work.).
After installing and testing the printer, use kmenu run and open xsane, the HP PSC 1600 Hundred should be in the title bar, test your scanner using either the preview or scan button, either should work.
If you wish you can also add xsane to kmenu. To do this right click on the kfor kmenu and select Menu Editor, in here right click on Graphics, or Menu Heading of your choice, select new item, name 'xsane', in the right panel click on the icon button and select the xsane icon. Next in the command box type /usr/bin/xsane then save.
In Kmenu Utilities you will find HP Device Manager, you will be able to configure scanner and printer, view ink usage and access photo cards.
Have fun :-D