To Add a Device Driver to an Offline Windows PE Image
In this step by step guide we are going to inject Windows 7 Intel LAN drivers to our existing boot image, I have an existing C:\winpe_x86 directory with the boot image (BOOT.WIM) and the LAN drivers (LAN folders) already copied.

1, Go to Start > All Programs > Microsoft Windows AIK > Deployment Tools Command Prompt > right-click and select Run as administrator

2, Mount the base image by using the DISM tool to a local Windows PE directory by typing the following command:
Dism /Mount-WIM /WimFile:c:\winpe_x86\Boot.wim /index:1 /MountDir:c:\winpe_x86\mount
3. Next is to add the .inf file(s) to the base image with the dism command with the
/Add-Driver option
Example: Dism /image:<path_to_image> /Add-Driver /Driver:c:\winpe_x86\mount\Windows\driver.inf
as for the actual driver:
Dism /image:c:\winpe_x86\mount /Add-Driver /Driver:C:\winpe_x86\LAN\Intel1\E1K6232.INF
repeat this step for additional driver.

4, After injecting the needed drivers we must unmount the image using the following dism command:
Dism /unmount-wim /Mountdir:c:\winpe_x86\mount /commit
Now we need to make a bootable media (CD-ROM or UFD) to test our boot image.
Free e-Book: Deploying Windows 7 Essential Guidance from the Windows 7 Resource Kit and TechNet Magazine

Contents of Windows 7 Resource Kit
- Deployment Platform
- Planning Deployment
- Testing Application Compatibility
- Developing Disk Images
- Migrating User State Data
- Deploying Applications
- Preparing Windows PE
- Configuring Windows Deployment Services
- Using Volume Activation
- Deploying with Microsoft Deployment Toolkit
Part II contents are from the Microsoft TechNet Magazine.
Windows AIK for Windows 7 appeared to have re-released last October 28, 2010 The only change is the version number to “Version 2.0”. No other changes were made in KB3AIK_EN.ISO that was released last July 2009.

For those who are using Windows® Automated Installation Kit (AIK) for Windows® 7 already, no need to download, for those who haven't here's the link: http://www.microsoft.com/downloads/en/details.aspx?familyid=696DD665-9F76-4177-A811-39C26D3B3B34&displaylang=en