This message is stored in the boot record of the drive, so if you get this, you know at least part of the drive is readable and uncorrupted.
This tells you that the bootstrap loader can't find IO.SYS. (for DOS/Win folk. I think the message is different for other OS's) Either it doesn't exist, or the root directory has gone screwy.
Much worse is the message Missing Operating System, which is a message from the system BIOS indicating that even the boot record is not valid.
Let's see the powerpuff girls save you then...