To mark the correct partition as ACTIVE:

From: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/dm_active_partition.mspx?mfr=true

Click Start>All Programs>Accessories>Command Prompt

At the Command Prompt, type this command and hit Enter:

diskpart

At the DISKPART prompt, type this command and hitting Enter:

list disk

Then select the disk you want (it's probably 0) by typing this command and hitting Enter:

select disk 0

Then list the partitions by typing this command and hitting Enter:

list partition

Select the partition you want to mark as active (it's probably 1) by typing this command and hitting Enter:

Then to mark the parition active we type this command and hit Enter:

active

You should get a message that the partition is marked as active. Now exit Diskpart by typing this command and hitting Enter:

exit

To mark the incorrect partition as INACTIVE

Issue this series of commands to list the disks and partitions, select the one you want to mark as inactive, and use the inactive command. In the example it says "n" but you need to put in the number that matches the disk or partition from when you "list" it.

Click Start>All Programs>Accessories>Command Prompt

diskpart

list disk

select disk n

list partition

select partition n

inactive

exit

Most of this info is from "Jim The Beam" on post: http://m-ntldrismissing.blogspot.com/2008/02/got-back-in-ran-bat-file-but-now-in.html

In XP, I accidentally marked a data drive active when trying to set the drive letter. I could not deactivate it. Neither could I mark my C drive where XP was installed as active.

I tried the solution of removing the data drive and it booted fine. But when I re-install the data drive it still has the ntldr is missing message. I tried using the XP repair console, but it ends up with the same bad message.

I did find a way to fix my problem. When booting the bios sellect F12 boot options. Then select master ide drive (instead of C disk). It booted fine. If I looked at my disks with the disk manager it shows the data drive as being active.