I tried installing Windows 7 Enterprise x64 on a Dell Optiplex 760. I used the Windows Media. The installation was so slow that I didn’t let it complete. I tried a 32bit version and there was no difference. I did some searching online and found a comment on Microsoft’s site that updating the BIOS should work.
I was using BIOS version A02. Updating to the latest version (A08) worked.
I then had a problem where the Windows 7 installer couldn’t see the physical hard drives. I went into the BIOS and changed the SATA mode from its default setting to something which I don’t currently remember and it worked. Windows detected the disk drives.
I’ve seen talk about installing the drivers and the problem goes away, but that’s not practical in manual pre-installation.