Building a PC from old bits

I was given a basic P166 PC minus hard drive. Installed an old 2gb drive as primary, cd-rom as primary slave. All jumper settings OK, mscdex copied to (formatted) HD, driver installed from a:, when I try to install the OS from d: (CD) I get a message saying “Invalid drive letter”. POSY detects the CD-ROM drive, but I can’t get it detected in BIOS setup. HELP