A peice of hardware can vanish if it doesn't get found when windows loads. But if it's because of an intermittent problem (bad ide cable bad ide port, bad drive, loose power cable) whenever the intermittent heals itself the next boot you should see the device drivers being loaded again...
1). Make sure your bios is set to auto detect all your drives. And if your machine is slow enough, make sure it shows the CD was detected during boot. The screen zips by too fast on XP1500 or faster machines.
2). Check to make sure the bios is set up to automatically detect plug and play devices. This will help if it is an intermittent problem. At least you will reload it automatically.