How to go back to 7

I let a friend use my computer and he thought he was doing me a favor by installing windows 8.
All of my important files are gone and I am stuck with windows 8 which I can't stand.
Is there a way to go back to the operating system I had before and recover all my files?
I am desperate to get to my files and return to normal.

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 8.1 Pro, 32 bit
Processor: AMD Turion(tm) II P520 Dual-Core Processor, x64 Family 16 Model 6 Stepping 3
Processor Count: 2
RAM: 2803 Mb
Graphics Card: ATI Mobility Radeon HD 4200 Series (Microsoft Corporation - WDDM v1.1), 320 Mb
Hard Drives: C: Total - 150368 MB, Free - 100195 MB; E: Total - 2040 MB, Free - 2024 MB; G: Total - 1035 MB, Free - 928 MB;
Motherboard: Hewlett-Packard, 1455
Antivirus: AVG AntiVirus 2014, Updated and Enabled