The SmartStep 100D is one of the few computers that Dell released that has a restore disk rather than a reinstallation disk. You should have two disks labeled 1 and 2. Set the computer to boot from the CD ROM drive by going to setup. Setup for this machine is restart and press the DEL key at the Dell splash. Navigate to the Boot order and make the change. Put the CD labeled "1" into the drive and allow the computer to boot. It should proceed on its own until it needs the second disk at which time it will stop and display a message to insert the second CD.

You mention three disks, but the third disk I had contained the drivers for things like the modem.

Good luck!