and make sure the first boot device is the optical drive. If not, make it so.
It sounds like the Master Boot Record is corrupt. If you can boot from the CD, go to the Repair Console (DOS-like screen) and type:
Agree to the warnings.
When I switch on I get a message :
"Windows could not start because the following file is missing or corrupt:
You can attempt to repair this file by starting Windows Setup using the original Setup CD-ROM.
Select 'r' at first screen to start repair."
This is a custom-built computer that is four-and-a-half years old, which packed up about 18 months ago with a spent motherboard. So the original constructor put in a new and bigger and better motherboard, and then it worked fine for about another year or so - then this happened about two months ago. Since then I have been existing on an older but much smaller machine, just to stay on line.
The problem is that when I switch on, with the OEM Windows XP Home CD-ROM in place, the computer does nothing, and the message remains on the screen. I have tried all possible combinations of putting the CD in the tray before and after switching on, and putting the disk in both my DVD ROM tray and my usual CD ROM tray. Nothing happens. The machine does not acknowledge the disk's existence.
I very much want to get the Windows repair option functioning first, to see if it can work. If I can't, or if the repair facility does not fix it - then I'm not patching this machine up again. I'll get the hard drive out and have another new machine built around it. That will be a lot of time and money. Who can help me to get this original system disk going, please?