Starting with Windows Vista/Server 2008 the OS installer creates a BDEdrive volume of about 300 MB containing the boot files. This BDEdrive is necessary to use BitLocker.

If you encounter a shortage of space on the C-drive of a Windows Server 2008 server and you add extra space to the VMDK file containing the C-drive, you'll notice that you can't extend the C-drive with the added space beccause the BDEdrive is in the way.
If you are not planning to use BitLocker you can follow the following steps to extend the C-drive:

1. Execute the following command in a command prompt:

   bcdboot c:\windows /s c:

   Output when succesful:

   Boot files successfully created.

2. Rightclick the C-drive in diskmgmt.msc and click on Mark Partion as Active.
3. Reboot the VM.
4. Remove the BDEdrive volume.
5. Extend the C-drive with the added space.

Source: Resources for IT Professionals > Forums Home > Solution Accelerators Forums > Microsoft Deployment Toolkit > BDEDrive Partition at the END of the disk after an MDT Deployment...