Go here and buy Windows 7 Pro here (Home edition is only a buck less so better to get Pro).
http://www.aakeys.com/windows-7-professional-sp1-product-key-sale-p-480.html
Once you get your Key Go here to download the Windows 7 ISO. You need your Key before you can download.
https://www.microsoft.com/en-us/software-download/windows7
Then download this tool here.
https://rufus.akeo.ie/
Put in a USB drive of 8GB or more, it WILL FORMAT IT, select the USB from the top drop down. Then where it says Chreat a Bootable Disk image select ISO image if not already selected. Then click on the CD Icon next to it. Find your Windows 7 ISO and then click ok. Then click start.
Once it is done, you can plug it in, not sure what the MSI GS60 boot menu key is but you either need to find it or go into BIOS and set either Removable devices as first boot, or sometimes they show up under hard drive boot order and set it to the first.
Then go in, boot from it, delete ALL partitions, and then install windows.
Not sure what model of the GS60 you have but i looked up the 6QE Ghost and if you click on the Service or Support right in the middle of the page and then go to drivers and then select your OS there IS Windows 7 64 Bit drivers there.
It doesn't look like you need to load a Controller drivers for the SATA Driver.
Also I would go in and download ALL the drivers BEFORE you wipe the drive in case you don't have access to internet due to lack of drivers. I would do this.
1) backup your data,
2) Download your drivers
3) make your USB Boot drive
4) Copy all your drivers to a folder called drivers on that USB Boot drive
5) install windows
6) install your drivers
7) Restore data
If you have any other issues let me know.
Also what is the reason for going back to 7? Their are a lot of benifits with windows 10 and gaming.