New internal hd

#newhd

Hello, apologies if this seems obvious. My hard drive is failing and I would like to install an identical replacement but I do not know how to install Windows on the newly installed  HD. (My pc is a Dell Alienware x51, Seagate Barracuda HD,1TB;  16 gb ram.)

I bought my pc from Dell about 3 years ago; did not get a dvd with OS. I would greatly appreciate some guidance as to how to get my pc up and running smoothly again with my new hard drive. I can physically replace the old hd and attach the cables but not sure what to do next.

thanks

thanks.