There are a number of reasons for this to come up.
1. It will come up when you first boot a new build and no O/S is installed. Completely normal and just stick your O/S disk in the dvd drive, set the boot drive to dvd in the bios and reboot. You can now install your O/S.
2. It will also happen if your pc cannot "see" your hard drive that you O/S is installed on. Go into the bios and ensure you have it selected along with the correct controller, AHCI (if applicable) etc. If it still cannot be seen change the cable and even try a different sata socket to rule out a faulty socket.
3. The files can be corrupted in which case you need to copy them from your Windows CD to your hdd with the O/S on. To do this you need to carry out the following:-
a. Insert your Windows disk into your dvd drive and select the drive to boot from.
b. When prompted, press any key to boot from the CD.
c. When you get to the Windows setup menu press "R" to repair Windows.
d. Log into your Windows installation by pressing the "1" key and press enter.
e. If you have an administrator password you will be asked for it now. Enter it and press enter. If you don't have one just press enter.
f. Copy the following two files to the root directory of the primaru hard drive. In this example we are copying these files from the DVD drive labelled "E". This letter may be different on your pc. These are the exact commands:-
copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\
g. when you have succesfully copied these files remove the Windows CD from the drive and reboot.
4. The Master Boot Record (MBR) could be corrupt in which case you need to repair it. Carry out a through to e as above and log into the recovery console. When in the console type FIXMBR and hit enter. When propted hit Y and enter again. Remove the CD and reboot.