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It sounds like your boot.ini, located in the root of the c: drive, is not pointing to where the operating system actually is. Edit the boot.ini and check to make sure the line reads something like this:
multi(0)disk(0)rdisk(0)partition(1)\WINNT="Windows NT Server Version 4.00"

It could also be that your CMOS configuration is set to ONLY boot from the A: drive. Go into it(F2 or Esc or F1 or something) during bootup and double check.

Good luck.

As strange as it may sound, try the video card drivers. Either get the lastest from the card manufacturer or find what chipset is on the card and get the chipset maker's drivers. If one doesn't work the other might. We've had this type of problembefore.

The boot disk you refer to...is it a floppy? If so it should contain the following files: boot.ini, ntdetect.com and ntldr, if you are booting from SCSI there will be another file you need. Copy them to the root of your boot drive (presumably C:)and try again. If that doesn't work, boot to DOS using a DOS diskette, and run the command fdisk /mbr to repair your boot record, copy the files again, and try again. Good luck.

With all the other answers here there might be one more thing that you might want to check and that is the size of the system partition. NT doesn't like anything above 7.8G for the system partition. Another thing to look at is the disk size. NT was made before the big platters so it doesn't really know what to do with it. I'm assuming this isn't the problem since you have NT on here. But if you have imaged it it could be a cause. Article Q197667. There is another article that uses several filesfrom SP4 and replaces files in an i386 setup directory that you make on the hardrive or network. This article ID# Q185773 describes what files to replace depending on what hardware that your using. I've had incredible success with this method and hope it helps.

Did you check your bios and the startup sequence. Make sure that the hard drive is in the sequence.