If this is windows, it sounds like file(s) in the boot sector are damaged and windows is asking for that Windows install disc be used to start the machine. If this is the case you may be able to start it up and use system restore to go back to a good windows state. Also if you can boot to safe mode you might be able to use the start with last known good configuration option. Another option (if you have the Windows disc) is to use it and choose a repair install. This will reinstall Windows and keep all your programs and files intact.