My hard drive is now partitioned to 1: (primary) 90% 2: (extened) 10% of drive i'll be installing windows xp. which partition do i make active?

The partition that XP is installed on will be made active automatically. You will have to change that later on if you want; but why would you? If a drive is not active it will not boot up. If it were me I would put the XP OS on the small portion and use the large portion as a library.

Remember you can not boot on extend partitions

Also remember: 1. You cannot mark a logical drive as active. 2. You can only mark a partition as active on a master boot record (MBR) disk. 3. A computer can only have one active partition per disk. 4. Changing or deleting an active partition might cause your computer not to start. 5. The system partition must be a primary partition that has been marked as active for startup purposes and must be located on the disk that the computer accesses when starting up the system. There can be only one active system partition on a disk at a time. You can have multiple basic disks and each can have one active partition. However the computer will only start from one specific disk. If you want to use another operating system, you must first mark its system partition as active before restarting the computer. 6. If you mistakenly mark a partition as "active" and it does not contain the operating system's startup files, your computer might not start. i_XpUser

I accidentally changed the partition of a secondary drive to active. I need to change it back, but there's no apparent way to do that in Disk Management. I have XP and it won't let me use Fdisk. is there any other way?