If you want to install the Windows 8 Developer Preview from a USB, you can follow the instructions below.
Download the Windows 7 USB/DVD Download Tool [USB Download Tool]
Download Windows 8 Developer Preview from Windows Developer Center. Make sure you are downloading the iso image. If you are not sure if your processor is 32-bit or 64-bit ready, get the 32-bit .iso file.
Insert a USB drive into your computer. (It must have 8GB or greater storage capacity ).
Install the Windows 7 USB Download tool you downloaded from the MS Store
Launch Windows 7 USB/DVD Download Tool you installed.
When it launches select the ISO file you downloaded earlier by clicking on “Browse”.
Select the USB Drive you inserted earlier. It must be 8GB or over and everything will be erased.
Let it run its course….
Once the tool is done, you will need to restart your PC with the USB plugged in.
On start up, you will need to enter the BIOS utility of your computer and boot from the USB. This is different for different computers. If you need help, just do a search for Entering Bios for your specific PC. In BIOS you need to make USB one of the bootable drives.
Once booted from the USB drive; follow on-screen instructions to install Windows 8 Developer Preview.