If you're dad gave it to you then I will assume that it is out of warranty.
Some of the DVRs have a warranty tape on the side so that you cannot open the chassis. The hard drive is inserted, and removed through the front panel.
This could be an AVC 770, or it could be a, 772, 773, or a 774
I am going to guess that you have an AVC 773. You should get close to what you need with this manual.
Code:
http://scorpiontheater.com/Documents/AVTech_Manual_English_AVC773_V1.0.pdf
If not open your chassis, and look at the model number next to where it will say AVTech, and post this model number.