Had the same issue on my gaming desktop. Only real thing I could figure out is that either your Windows 7 build is broken or your hard drive is dead/dying.
If you have a Windows 7 recovery disk, try using that first; open the disc drive, press F11 while on the manufacturer logo (there should be a list of options upon startup on this screen, if not, default boot menu key on most Windows 7 builds is F11) and select the option for the disc drive boot.
On mine, this was A1 (A0 was the hard drive) and what should happen is that a screen will pop up for around 2-3 seconds - it should say "Press any key to boot from CD or DVD". Press a key, and eventually (it took me about 2 minutes) a list of recovery options should pop up. First one in the list should be Windows 7 Boot Repair (or something to that effect.)
Also check your BIOS (should be F2 when first starting your computer, although I've seen a few people map it to F6 or F7 for some weird reason) and see if the hard drive is viewable, as in the BIOS detecting it being there.
Not sure if I'm 100% accurate, as I have no idea what the common brands like Dell and HP have during startup (I use an ASRock motherboard in my desktop.) Hope this helps.