Is this happening when you try to boot your PC? If so, then there are two possibilities:
1. Your Master Boot Record (MBR) is damaged, or
2. You have a CD or DVD in your PC's DVD drive that is not bootable.
Can you tell us which one it is - you can check by removing any USB external storage devices, and maiing sure you have no CD's or DVD's in your CD/DVD drive. Can you also tell us whether you have an OEM Windows installation, or whether you have a Windows installation DVD?