Prepping the USB Drive
Now that your slipstreamed installation files are ready, it's time to prepare the USB flash drive. Any data currently residing on the drive will be deleted during this process, so back up anything you want to keep. You'll need at least 4GB on the drive--and the faster it is, the faster the install process will go.
First you'll need to make the USB drive bootable, which you can do with Windows 7's built-in Diskpart utility. Plug the USB drive in and then open a command-prompt window with elevated privileges by typing CMD in Windows 7's search field and pressing Ctrl-Shift-Enter. In the command-prompt window, at the C:\ prompt, type Diskpart and press Enter to launch the utility.
At the DISKPART> prompt, type List Disk and press Enter to see a list of drives available in the system. You must select the USB drive before continuing; on our system, the USB drive was labeled Disk 2. Type Select Disk x (replacing x with the number of your USB drive) and press Enter, and the drive will be selected. At the DISKPART> prompt, type Clean and press Enter to wipe the USB drive of all old data and partitions.
Next you have to create a new partition, make the partition active, format it using the FAT32 file system, and assign the drive a letter. At the DISKPART> prompt, type Create Partition Primary and press Enter. When the next prompt appears, type Active and press Enter. Finally, type Format fs=fat32 quick and press Enter. When the formatting process is complete, type Assign and press Enter one more time, and the drive will be ready. Type Exit, press Enter to close Diskpart, and then close the command-prompt window.
With the USB flash drive prepared and ready, simply copy the slipstreamed Windows 7 installation files over to the drive, and you're technically ready to perform an installation. We have one more quick tip, however: Once you have copied the Windows 7 installation files, explore the drive, and in the /sources/ folder, rename the file labeled ei.cfg. The ei.cfg file tells the Windows installer which version of the OS to install by default. With that file renamed (or deleted), the Windows installer will prompt users with an additional menu and allow them to select from Windows 7 Home Basic, Home Premium, Professional, or Ultimate.
The Install Process
To perform an installation from your newly created, slipstreamed, bootable USB flash drive, insert the drive into a PC and set the system's BIOS to boot from it. Restart the machine, and the Windows 7 installation will launch as if it were running from the original DVD--only it will load files much faster.
How much time will you save? Our test flash drive with the slipstreamed installer took 6 minutes, 35 seconds to go from the 'Windows is loading files' prompt to the 'Completing installation' prompt. Using the stock Windows 7 installation DVD, the same process took 14 minutes, 30 seconds, and that doesn't include the additional time necessary to download and install SP1--which could be hours, depending on the Internet connection.