During the default installation of Windows 7 a 100MB System Reserved partition is created that supports BitLocker encryption and protects the boot environment from inadvertent changes.

It is possible to deploy Windows 7 without this partition, but BitLocker cannot be enabled later.

When deploying Windows 7 in an unattended manner there may be a requirement to format the drive first, possibly to allow additional installation files, such as network drivers, to be copied to the local drive.

The following diskpart commands will create partitions that will allow Windows 7 to install with a System Reserved partition on a pre-formatted disk:

SEL DISK 0
CLEAN
CRE PAR PRI SIZE=100
SEL PAR 1
FOR FS=NTFS QUICK
ACTIVE
CRE PAR PRI
SEL PAR 2
FOR FS=NTFS QUICK
ASSIGN LETTER=C:

Note: The System Reserved partition can be increased and hold the Windows Recovery Environment.

Avoid specifying the specific InstallTo partition in the unattend.xml. Instead specify InstallToAvailablePartition:

<ImageInstall>
    <OSImage>
        <WillShowUI>OnError</WillShowUI> 
        <InstallToAvailablePartition>true</InstallToAvailablePartition> 
    </OSImage>
</ImageInstall>

Recommended BIOS-Based Disk-Partition Configurations
http://technet.microsoft.com/en-us/library/dd744364.aspx

Turning On BitLocker Drive Encryption on an Operating System Drive
http://technet.microsoft.com/en-us/library/ee424299.aspx