Number of things. Usually the system is unable to find the boot files, which can be caused by a virus, bad sectors on your drive, disconnected drive, bad cables, bad power, etc.
Installing new hardware can do it. Alogn with changing any part of the wire management when it comes to the hard drives. IF you have them on IDE cables and you changed the placement on the IDE cable you could have changed yoru config. Could have put it on the Slave part of the cable instead of the master.
Connection 1 is considered the Master connection. This will be the Master drive. Connection 2 is the slave. So if you put the hard drive on a different part of the cable you would get this error.
Okay. What OS? Coudl be a driver malfunction. Could be several things when changing hardware. Even a video card. I have seen Vista ask for reactivation with a simple replacement of a video card.
OK, I inserted the DVD and booted up windows. I went to repair, and no operating systems show up in the list. Do I go to load a driver from the CD? If so what and where is it?