You dont need a license for Win 8.1 to install 8.1. Win 8.1 was a free upgrade. You need a key to install 8.1 and then you can use your 8 key to activate. Microsoft themselves provide the install keys.
This is official and from Microsoft, im not providing anything illegal:
You just need a Windows 8.1 iso which can be gotten from the official Win 8 download utility.
Use the above at your own risk.
Run any and all diagnostics you can, test mem, gpu, cpu, hdd's.
make sure all firmware is up to date.
Did you install Windows in Legacy mode or UEFI mode? (google this, not the same as your bios being UEFI/LEgacy, tho related)
In eventvwr go to Windows Logs and check application and system, report the exact errors (red icons) making sure to include event id, any error codes, and exact description. Error 41 was likely from you manually restarting. The events have time stamps, using those will help you pinpoint what you know happened when and whats going on around it. IE you manually restart at 4pm, error 41 shows at 3:59:30pm and theres some other error at 3:57:00pm then that error is likely the issue.
Are you using the nvidia graphics driver from nvidias site, the graphics from the card manufacturer or from Windows update? regardless try one of the others you havent tried yet.