Actually very good article provided by Chaohao Xu

The Windows Automated Installation Kit (AIK) for Windows 7 SP1 is released and can be downloaded from the following location:
http://www.microsoft.com/downloads/en/details.aspx?FamilyID=0aee2b4b-494b-4adc-b174-33bc62f02c5d

You don’t have to upgrade to Windows PE 3.1 to deploy Windows 7 SP1 and Windows Server 2008 R2 SP1. But if you want to upgrade to Windows PE 3.1, this post provides information about how you can do this with your existing Configuration Manager 2007 SP2 site. We also list some known issues that you might encounter when you use Configuration Manager operating system deployment with Windows PE 3.1.

Upgrade Steps:

1. Use the following instructions to upgrade to Windows PE 3.1: from http://go.microsoft.com/fwlink/?LinkId=139690.
2. Add a boot image based on Windows PE 3.1:
   1. In the Configuration Manager console, navigate to Computer Management -> Operating System Deployment -> Boot Images -> Add Boot Images.
   2. Specify the UNC path for the Windows PE 3.1 .wim file and select the image from the WIM file.
   3. Use the wizard to create the x86 and x64 boot images.
3. Update the source of the default boot image to Windows PE 3.1 by running the ExportDefaultBootImage() WMI method:
   1. WMI Namespace is rootsmssite_<site_code>
   2. WMI Object is SMS_BootImagePackage
   3. WMI Method is ExportDefaultBootImage
   4. Parameters:
      • Architecture (x86 or x64)
      • ExportImagePath (\<site _server>sms_<site_code>OSDbooti386boot.wim or \site_server>sms_<site_code>OSDbootx64boot.wim)
      • ImageIndex (1)
        
4. Update the distribution points for the default boot images.

Known Issues:

1. Configuration Manager does not support using Windows PE 3.0 as a boot image after Windows PE 3.1 is upgraded. This means any boot images, including the default boot images, that are based on Windows PE 3.0 cannot be updated in Admin UI anymore. In this scenario, you will see the following error because the Windows PE 3.0 images are not compatible with the optional component (OC) and language packs from Windows PE 3.1:
   
   To resolve this problem, update all the boot images to use Windows PE 3.1.
2. Task sequence fails when trying to apply drivers. You might see the following errors logged if task sequence fails to apply drivers.Failed to find the certificate context.. Cannot find object or property. (Error: 80092004; Source: Windows)
   Failed to initialize the online driver catlog.  Code 0x80092004
   Failed to auto provision drivers.  Code 0x80092004
   Failed to run the action: Apply Device Drivers. Cannot find object or property. (Error: 80092004; Source: Windows)This problem is caused by the Configuration Manager 2007 client generating an embedded NULL character into the friendly name for the certificate that is related to the driver apply. To resolve this issue, see KB977203 and run CCMCertFix.exe either before you run the task sequence or add it as a step in the task sequence.