What’s that mysterious 100-MB partition in Windows 7 ?

If you install Windows 7 on a clean disk with no existing partitions, it creates a System Reserved partition of 100 MB at the beginning of the disk and uses the remainder of the unallocated space to create your system drive. That small partition isn’t assigned a drive letter, so you won’t even know it exists unless you look in the Disk Management console (as shown here) or use Diskpart or a similar low-level utility to inspect the disk structure.

This stub of a partition, new in Windows 7, serves two functions. First, it holds the Boot Manager code and the Boot Configuration Database. And sec¬ond, it reserves space for the startup files required by the BitLocker Drive Encryption feature. If you ever decide to encrypt your system drive using BitLocker, you won’t have to repartition your system drive (a genuinely tedious process) to make it possible.

If you’re confident you’ll never use BitLocker and prefer to do without the additional complexity of this 100-MB System Reserved partition, your best bet is to make sure it’s never created. For a truly clean installation starting from an unformatted hard drive, you must use an alternative disk-management utility, such as the setup disk available from many hard-disk manufacturers or a startup disk from Windows Vista. Create a single primary partition using all unallocated space, and then point the installer to the newly created partition as the setup location. Note that you cannot use the graphical disk management tools available from the Windows 7 DVD to perform this task. After you use the third-party tool to create a partition on the drive, you can point the Win¬dows 7 installer to that location and it will proceed.

If you’re comfortable with command-line disk management tools, you can use the Diskpart utility from the setup program to create the necessary partition. At the begin¬ning of setup, before you select the location where you want to install Windows, press Shift+F10 to open a Command Prompt window. Then type diskpart to enter the Diskpart environment. Assuming you have a single clean hard disk, use select disk 0 and create partition primary to manually create a new partition, which you can then use as the setup location.For a more detailed discussion of Diskpart, including some caution¬ary notes.

Source of Information : Microsoft Press - Windows 7 Inside Out