The problem isn't so much installing the new hard drive physically, but the fact that you'll need to reinstall Windows to get it to work. Your system (I'll assume it's a Dell) came with a restore disk that'll put your system back to the state it was when you bought it.
Once you open up your computer and replace the hard drive, you'll then need to use that restore disk and then get Windows Update to install all the patches, which could take a while especially if you have a slow connection.
I'd find out how much The Geek Squad (or some of their competitors) would cost. It's not difficult, but it can be time consuming. How much is your time worth?