How can i burn my Windows 8 To a USB?

I would recommend you put the new hdd in and use the live cd (clonezilla) to copy the hard drive over to the new one before it fails. If it's a retail computer contact the company and they can give you a set of restore disks which will restore it to factory default. I would recommend you backup your most important files to your usb immediately.
 
http://www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/
use a usb stick and microsoft iso to usb tool make a new usb boot disk. you need a 8g usb stick. the boot disk will be clean of any of the old disk errors. after you install the os onto the new drive and a anti virus software plug in the old drive and copy your data over. if the drive and windows is bootable now use google drive and back your files to the cloud.
 


That only allows you to put the installer on a usb bootable thumb drive, it wont take a currently installed windows and create an installer from it. I believe he has a retail system with windows preinstalled and the typical restore partition. I don't believe he has a set of restore disks.
 


Hopefully he has a working pc to do this on, I wouldnt touch the thing till he had the second hard drive and just try to do a drive to drive copy if possible. He could also use a tool like clonezilla to make an image of his disk and restore it later that way.
 


http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media

It will walk you through the download and creation of a usb installer

I would backup your files as this will wipe your system when you do the install. remember not to backup to the same usb pen drive you plan on using for the install as it will most likely require a format.
 


Cloning from a HDD that is failing/having problems usually ends badly. A clean install is the way to go. Less possibility of problems and wasted time.