And another explanation:
Originally Posted by chipstone
When you use Windows Setup to create a new drive and you choose to use all the available space, Setup leaves approximately 8 MB of un-partitioned space on the master boot record (MBR) so you can convert the drives to a dynamic volume.
This space is automatically reserved when the partition or volume is created in Microsoft Windows 2000 or Windows XP Professional.
I am not sure why, since you cannot boot from a dynamic disk if you choose to convert the boot-drive to dynamic. However, you cannot by-pass this functionality