Hum... Could be a problem with the OS, with the hard drive, or with the RAM, idk.
It's a desktop computer, not a laptop right ?
You can try :
- Making a Linux bootable USB key (if you have access to another windows computer, just download Lili USB Creator, download a linux image (.iso) like linux mint for example : https://www.linuxmint.com/download.php then plug your USB key (be aware that it will be flushed), run Lili USB, and tel it to put your .iso file on the key)
If it works the problem can't be the RAM.
When you're at it copy your files on an external disk or something