Putting old hard drive in new computer??

I just bought a new PC for a client of ours because her other one was ruined due to static discharge. I hooked up her old hard drive in the new pc, changed the master/slave settings so I can boot from the old drive, but when I start the computer, after all of the hardware is initialized (it is recognizing both drives), I just get a blinking cursor and it will not boot. When I change the master/slave settings back to boot the new hard drive, everything works. Are there any settings that I am missing that is not allowing it to boot from the old drive. The information on that drive isnt corrupt, as I can still run programs from it. Thanks for your time!!!