Desktop 8.1 computer completely crashing

[Lyil]

After down loading windows 10 once I'm past the sign in start page every thing is just flashing and the computer is totally unresponsive. It is impossible to access any programs. I can't even try to go back to 8.1 unless I can do something in the UEFI BIOS setting I'm totally sunk.
I must admit to being a little naive as I didn't realize windows 10 was an experimental program for enthusiasts only, being as the download symbol had appeared mysteriously on my monitor, and the tv is showing advertisements for windows 10 every hour, I assumed that I wasn't going to bugger up a 3 month old computer.;

 

[BigFeet]

Put your windows 8 media in your PC and reinstall Windows. There shouldn't be anything you have to do in your bios.

 

[Lyil]

Thank you for the help Bigfeet but unfortunately I have no Windows 8 media to put in so I can't reinstall anything, so what can I do instead?