Find driver.cab on your Windows XP install CD. It should be in the
\i386\ directory. If you have purchased a computer with Windows
software pre-installed, search your hard disk for this file. Computer
manufacturers often place this and other Windows install files in a
directory like c:\msoft so that it's easy to add more Windows
components later.
Unzip this file to your desktop in a folder called DRIVER, using
an utility such as 7 zip. It's large and will take a while.
Start the installer for the HP 8500a software again. Once it's
found your printer over the network and asks you for serscan.sys and
sRGB.ICM, navigate to the DRIVER folder you unzipped to your desktop.
The installer will continue.