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:
Open up a command prompt and type DISKPART.
Type LIST DISK
Type SELECT DISK n (where n is the number of [...]