|Each SATA controller usually controls TWO SATA ports. So, your SATA controller should have already been enabled or the original drive wouldn't be working.|
Because this is a laptop things are different. You can't add more hard drives other than the one additional drive you evidently have added. That shouldn't be a problem.
Try removing the original drive temporarily and then installing WinXP on the new drive.
Your second drive bay may have been intended for use with a RAID array. There are a couple of different RAID arrays possible with 2 hard drives.
My guess is the default is to allow RAID 1, which is mirroring of the original drive to the second drive. This is done to protect data.
Enter the BIOS (setup) screens and look to see what options you may have to setup the two drives. I would assume you can run them independently. That may require a change in the BIOS settings. The manual should also cover it.
As I mentioned above a boot manager may be the best route.
Boot managers allow totally independent operation of any and all installed OSes. Windows boot manager depends on the legusy OS to control the subsequently installed OSes. This, in my mind is not a good setup. If the legusy OS becomes trashed you may not be able to access the others.
A third party boot manager requires a small partition at the front of the primary drive.
I suggest you swap the new drive for the Vista drive. Partition the new drive as required by the third party boot manager. Then install WinXP on the second partition of the new drive.
Boot the laptop without the Vista drive connected and verify all is well and the boot manager is functioning. Then reconnect the Vista drive and the boot manager should be able to find the Vista OS.
Before performing ANY of this be sure you have created the Vista restore disks, unless they were shipped with the laptop.