Although I don't have an HP5100c scanner I do have an HP4100c that is
about the same age. I upgraded computers from a Windows 98 to a Windows
xp machine about 6 months ago and had the same problem, couldn't load
the original software. Someone here suggested that I go to HP's web site
and download the Windows 2000 software for my scanner and also the
Windows xp update for it. I downloaded the software from the web site,
installed the Win2000 software, updated it with the Winxp update and it
has been working, although it still had some problems (i.e. I would get
an error message occasionally that said "unidentified error"). When the
error occurred I would shut down the HP Precision LT software and start
it again and it would usually work.
My kids bought me a new Microtek 6000 scanner, because of the occasional
errors, and I found out that the USB port that the scanner was connected
to had intermittent errors with the new scanner. I connected the new
scanner to a different USB port and it worked fine. So I connected my
old trusty HP 4100c to the different USB port and it as been working
fine ever since (about one month).
Check on HP's web site, download the Win 2000 software, and the Win xp
update for your scanner. Then install the Win 2000 software, update it
with the Win xp update and see if that works.
Good Luck!!!
Bob