Oh. That kind of active.

Grab a Linux boot disk, boot off of it, then run the "fdisk" command against the appropriate drive. Assuming it's the second IDE drive in your system, probably "/dev/hdb". Type "p" to show the current partition table. There should be an asterisk in the "boot" column on the partition you marked active. Type "a"; it'll ask you for a partition number. Enter the partition you want to toggle the flag on. Type "p" again to verify that it's gone. Then type "w" to write it out and exit. Reboot.