can’t upgrade nt

I’m trying to upgrade Windows NT from an evaluation copy, to a licensed copy. My problem comes in when I run the 3 setup disks. After disk 3 has run, I get the message “NT could not find a hard disk. Setup will not continue”. However, the trial version is still able to load and run for about an hour before shutting down. The strange thing is that this so called trial version was pre-loaded on the machine and has run without major problems for two years. My server is a Compaq Proliant ML350.