How to mark a partition as active using the Windows interface:
1. Open Computer Management (Local)
2. In the console tree, click Disk Management.
Where?
Computer Management (Local) > Storage > Disk Management
3. Right-click the primary partition you want to mark as active, and then click Mark Partition as Active.
If you managed to go into Disk Management in Windows XP and somehow click “Mark Partition as Active” accidentally, this could render your computer unbootable. If you are so lucky that you haven’t rebooted yet, here is one possible solution to the problem: mark the partition as inactive using the DISKPART tool:
If you have problems booting your computer or the above approach didn’t work for you, you could also try what Microsoft suggests on their help page entitled The computer does not start after you change the active partition by using the Disk Management tool – external link, opens in new window. For your convenience, the content of that page is reproduced below:
