Computer randomly shuts off - More details below?

Pc specs I could find:

Processor: AMD Phenom(tm) II X4 810 Processor with 2.60 GHz

RAM: 8.00 GB (7.75 GB usable)

System type: 64-bit OS

I have windows 7 ultimate but I've have had it since 2009 and it's now also started saying it's not genuine and I need to upgrade to a better version.

When I play games that take up a bit of RAM or are bigger games like LOL (League of Legends) Minecraft, or Runescape it shuts off. Then after it shuts off it takes atleast 20+ tries to start it up again and get past the user login screen, where I choose my user or main account. Once I try it shuts off again and again. I've cleaned a lot of space from my pc so it has a lot of GB, and the dust from inside my computer. But when I get home after school the pc turns on fine and then once I run Minecraft for 10 minutes it shuts off and then the problems begin. This problem has affected my Youtube channel and my upload times. If you need more details tell me how to get them. Also you may have to explain the answer more to me because I have low intelligence about the inside of computers and that stuff. And also I can't set my pc back to a saved checkpoint because my Mom is doing online classes and has a lot of Microsoft Word documents saved for all of her assignments so if you can tell me how to protect them if I do roll my pc back to the checkpoint before all this happened.

You most likely have to erase, reformat your hard drive and reinstall your windows