My Computer is toasted. I need to re install Windows 10? I have the flash drive but don't know how to use it? Can I install Windows 10 and keep the program? Or do I need to build the computer from scratch?
You will want to format the hard drive and reinstall Windows from scratch.
If you have data you need to backup you're best booting into a recovery environment to copy it to an external drive. Use a utility like Hiren's. https://www.hirensbootcd.org/download/
It depends what you mean by my computer is "toasted". This could either be a hardware problem or windows corruption.
You can try repairing windows first before a clean install. Boot of the flash drive, choose the repair option bottom left corner,
Then Troubleshoot>advanced>startup repair. If that fails try an in place upgrade
Pick the install now option instead of the repair option and try an upgrade install.
If that fails you would have to do a fresh install. In either case try to use Hirens to copy off your data first