Common causes:
If you have a floppy drive, make sure there isn't a disk in it.
Your computer isn't seeing your hard drive. Check in your BIOS screen to make sure your computer sees your drive. If not, check/replace the cable and check the power connection to the drive.
If you have multiple drives, make sure that your BIOS is set to boot from the one with the OS installed.
If you are booting from a RAID - did you install the right drivers during setup?
Was it working before? If so, you could have a corrupt bootloader - you should be able to fix this with a repair, rather than a full re-installation. - Try booting from the windows DVD.