Use the Microsoft-Windows-PnpCustomizationsWinPE answer file section to add drivers to the winPE pass.
Use the Microsoft-Windows-PnpCustomizationsNonWinPE answer file section to add drivers during the offlineServicing pass
And if you need to add non-boot-critical drivers later during the installation process (for example, a video driver) then you can use the Microsoft-Windows-PnpCustomizationsNonWinPE answer file section to add them during the auditSystem pass.
Mitch Tulloch was lead author for the Windows Vista Resource Kit from Microsoft Press, which is the book for IT pros who want to deploy, maintain and support Windows Vista in mid- and large-sized network environments. Mitch was also the author of Introducing Windows Server 2008 and technical project lead for the Microsoft Office Communications Server 2007 Resource Kit, both books also from Microsoft Press. For more information on these and other books by Mitch, see www.mtit.com.