When creating an OSD build, the standard method for deployment is to capture a reference machine using a Build and Capture. You may want to avoid using a Build and Capture method to avoid any drivers being installed in your captured .wim file.
So why not use the install.wim file from the Windows 7 media? Makes sense right? The problem is, if you use this wim file to deploy other machines the default drive for the OS will become a D drive rather than C. How annoying!
Apparently with Windows 8 this problem is resolved but for Windows 7 luckily there is a workaround, if you are using SCCM 2012 SP1 that is!
Open up your Windows 7 task sequence and before the Apply Operating System task add a new task. Click Add>General>Set Task Sequence Variable.
Enter the Task Variable name OSDPreserveDriveLetter and set its value to FALSE.