If your son can build computers, he can surely help you with transferring data files. You will need install files, cd or downloaded, to install your programs to the new computer.

My preferred method is to network the two computers. Plug both into a router, switch, or hub and configure a network. Share the folders that have the data files you need, click and drag them to new computer. This is also a great way to back up data and to have a backup computer that keeps you in operation if your new main computer gives some problems.

A second method is to install the hard drive from the old computer, as you said, as a second hard drive (slave drive) to new computer. Of course, this prevents you from using old computer as a backup computer.

Jump drives are a pretty effective third method, fast and easy, too, just not quite as convenient as networking.