I just bought one and had the same error and had to spend 30 min on hold with ATI (long distance! no 800#!!) to get this question answered. The problem is that the drivers are not configured to properly recognize the video bios. Stupid software error by ATI. You can still install the drivers however and they work fine, just you can't do the autoinstall.
My OS is win98se, so this my differ slightly.
Manually install the drivers by going to the display adapter in the system menu in the control panel. Click on you video card (mine still read PCI adapter (vga) at this point) and click on "update drivers" Then you will have to manually select the video drivers you wish to install. I unpacked the drivers I downloaded onto my hdd, and the file for my OS read something like "atii98x***.inf" Begin to install this file and the rest of the files will auto install.
Make sure you uninstall any old video drivers and you have booted with the general pci vga adapter (and you comp will start with 640x480x16) and have installed directX 8.x before you start. Also make sure it overwrites any driver files still found (ie it will sometimes ak if you want to keep a newer file and just say no.)
Good luck, and hopefully in a few mins you will have a working 7500.
PS. the tech guy said this is a common prob. ATI is dumb for letting it happen in the first place, but dumber for not putting this on thier website. Oh well, it kicks ass once it works.