I want to install Windows 8 on my tablet but do not want to dig around to find my DVD player, so I decided to grab an old 4gig USB key and creating a bootable install for Windows 8.  There are two ways to do this.

The easy way is to simply download the Windows 8 Consumer Preview tool.  This tool will check that you have at least 4 gigs free on your machine, check to see what apps will or will not run under Windows 8 and finally allow you to create a partition, DVD or USB installable disk.

Now, if for whatever reason you cannot or do not want to take the easy route, you can download your own copy of Windows 8 and the manually create a bootable USB drive yourself.

First, get a copy of the Windows 8  iso file you want.

Next,

1. First, you need a USB Flash drive (at least 4GB – any less and you can stop reading right here).
2. You also need the Windows 8 Consumer Preview .ISO file – 64-bit download | 32-bit download
3. You also need to download and install the Windows 7 USB/DVD download tool available from Microsoft.
4. OK, now you’re ready to rock. Pop the USB flash drive into a spare USB port and fire up the Windows 7 USB/DVD download tool and browse to the .ISO file you want to use, and then click Next. 
5. Click on USB device as the media type.
6. From the next screen choose the correct USB flash drive from the drop-down box and then click Begin copying.
7. The tool will now format the drive and then copy the Windows 8 installation files onto it. This will take some time, so be patient!
8. Once the tool is done copying the data across, you’re ready to use the USB flash drive to install the Windows 8 Consumer Preview onto your hardware. When you’re ready to install Windows 8, insert the USB flash drive into a free USB port, and then run Setup.exe from the root folder on the drive.

That should do it.