When you use Disk Management to mark a primary partition active, the computer does not start up if the the active partition is NOT the System partition, the one that contains
NTLDR NTDETECT.COM BOOT.INI.On Intel based computers, the System partition must be a primary partition that is marked active. There can only be one active System partition at a time.
To fix the problem, do any of the following:
1. Use any DOS based utility that can change the active partition.
2. Perform an alternate install and use it to make the System partition active.
3. If the current active partition is properly formated, use the Recovery Console.
NOTE: The active partition is always considered to be the C: drive, so when you boot the Recovery Console, you need to copy the above files to the root of this new C:. If the partition is not properly formated by Windows NT/2000, you may need to use the Recovery Console's FIXBOOT command to make the active partition bootable.