Replacing the hard drive is not difficult. The problem is that you need to install windows and other software, and you may have lost all your documents and pictures etc.
You can assume it was a 2.5" drive. You need to find out if it was IDE/PATA or SATA. There will be a ~3"x4" door on the bottom of the laptop held by 2 or 4 small phillips screws. Open this to access the hard drive.
If you are not handy yourself, it may well be worth enlisting the help of an expert. It's even possible that they could recover your hard drive or some of your files.
Most Laptop hard drives are lost to dropping the laptop, even a few inches. Do not drop a laptop!