If you don'yt get any responses on this from people that know a quick fix, i would suggest a couple of things you might do to proceed.

There's a couple of question you should also list here:

1. Was this working before with your present computer configuration? (i.e. the same windows version, etc)

2. If not, you should definately check HP for a patch and or update.

~First and foremost I would try reinstalling the driver from your cd.
~Check the HP website and see if there is a fix, patch or update.
~check to see if you can trigger the slide function from within the HP scanner software. I had a problem with the copy function of my HP scanner, where the button on the scanner didn't work, but I could access the function through the software.