Either your loading from the wrong hard drive or your Windows installation is corrupted. I would check your boot sequence and make sure that the hard drive with the OS on it is listed first.
Then if it is you will need to put your Windows cd in and do a repair so that it loads the BOOTMNGR file and your Windows will then boot into the desktop.
How long have you been using the computer with this operating system installed ?
Is your omputer a desktop or laptop ?
Just to add to this for future User's experiencing similar-related boot-up
Normally when User's get: "Boot-manager cannot be found" during reboot/start-up -is due to the incorrect: Disk Priority Device being set within you're Bios Settings.This Error-Message can occur when unplugging/changing Sata-Ports on your Mobo as well. On Win7(depends on one's Bios Manufacturer mostly)press Delete-Key(yours may differ)repeatedly right when rebooting.
Then choose Advanced Bios Features/or Similar Tab. Once there click Advanced Disk Priority BootSequence.Now, click Enter on the first one & choose enable on whichever is your Main Disk/OS to boot from. Click F10 key on keyboard to Save and Exit to reboot.
*Just a note:
If you have no disk for your OS and have an OEM OS installed into your Computer,one can click the F8 Key(yours may differ)to Repair Computer, or select: Last Know Good Configuration that Worked & etc., Options.