If your old hard drive is a sata drive then yes it will run, but if its SUPER old and is IDE, then it will not run with that board natively, although IDE to sata adapters are fairly cheap.
What I don't understand is the part about the OS. Even if you get a new hard drive, why wouldn't you be able to keep the OS? If you have a retail copy, then you can use it as many times as you want assuming that it is only being used on one system at a time. If it is an OEM OS disc, then legally you shouldn't be re-using the license after you swap parts with it as OEM licenses are tied down to the system.
This is of course, speaking legally *cough*
