This is what I found on an online computer course that I am taking:
When the computer passes all hardware tests, the BIOS looks for an operating system. The PC first looks for an operating system on the floppy disk. If you get this error:
Non-System or disk error
Replace and press any key when ready
You have probably left a non-bootable floppy disk in the floppy drive! Removed it. If you do not have a floppy disk inserted, the BIOS looks for an operating system on the first cylinder and first sector of your hard drive, a location known as the "boot sector." The vast majority of all PCs store two files on the boot sector of the hard drive: IO.SYS and MSDOS.SYS. If one of these files is damaged or missing, your computer will fail to start. You will then see an error on your screen, such as "Invalid system disk" or "Missing Operating System."
If IO.SYS and MSDOS.SYS are in proper working order, the computer then searches for your COMMAND.COM file. If this is missing you will get an error, such as "Bad or Missing Command Interpreter."
All of these errors are trivial to fix, assuming you have your rescue disk handy. Simply insert the disk and reboot the system to boot off the rescue disk. At the A:\> prompt, type:
SYS C:
This automatically reinstalls a new copy of the IO.SYS, MSDOS.SYS, and COMMAND.COM files. Make sure you use the correct rescue disk! Do not use a DOS rescue disk on a Windows 98 system, for example, as the DOS rescue disk would place DOS copies of these files—the result would keep the system from booting!
If this doesn't work, email the tech support people supporting the troubled computer: e.g. Compaq tech support, HP tech support, etc. They are a great resource. You will need to complete a form stating the problem, along with the serial number for that computer. It usually takes 24 hours to get an answer.
