well first, is there a disk in the floppy drive? try removing it, then rebooting.
that message means there's no OS on the first disk drive of the "boot sequence."
if the error is not because of the bios looking for an OS on the floppy, even if your hard drive operating system files are corrupted/gone, with a little perseverance you can remove the drive and hook it up to your other machine and get the data files off it that way-- assuming of course, that the drive still works. - fj
I have an old PC (Compaq Presario 4910) that I haven't used in years. Today, I realized I might have some old files on it that I might want to see again, so I hooked it back up. Unfortunately, after the standard COMPAQ screen and password check, I received this error:
Invalid System Disk
Replace the disk, and then press any key
I remember that in the days leading up to the purchase of my current PC, I deleted a lot of excess files, but I didn't think I did anything to hurt my computer. Maybe I was wrong. What happened? What does "Invalid System Disk" mean? How can I fix it?
Thanks in advance.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic