A while back I had two hdd's setup, one as master and the other as slave. Upon restarting my COMPUTER nothing would load, so I simply removed the primary drive and SET the slave as master.

Yesterday I tried installing Vista and started to get some serious errors, so I figured I'd set the drive back to slave and bring my older one back as master. When I load up the computer with this setup, it only detects the master drive and says nothing is there for the primary slave IDE slot. The hdd itself isn't burned out as it worked just the other day. I think I am experiencing the same problem as before, but I can't simply go back to the ORIGINAL setup because I need to pull some data off the fubared drive.

Basically I'm wondering why it won't detect the hdd as slave. The jumpers are in the correct spots respectively, the cables are plugged in properly, auto detection is ACTIVATED in BIOS... what could be the culprit?Do a few cold boots with just the master hooked up.
Then power down double check all connections and jumpers.
The master drive should be at the end of the IDE cable with the slave in the middle.
Re-boot and see if this helps...it is highly likely that the mbr is still on the slave drive and therefore it will still cause problems even when jumpered as a slave. disconnect the old slave and only use the drive you have designated as your master for a few times (as per " Patio's " instructions) the mbr could be changed during use as the master, then after a few boot ups TRY with the slave drive attached.
or
do a repair installation!
or
it could be fixed using fdisk /mbr if you know how to make a windows floppy, but this all depends on if you have any partitions on the drive, normally fdisk is done during the installation of XP and you will not even see it happen. if you have no partitions there is really no problem about using FDISK /MBR.
Look at this for a guide.
http://support.microsoft.com/kb/69013
fdisk /mbr can be done from the recovery console
or
please note the FDISK for XP is different from other versions of windows, you can create a floppy by going into mycomputer then put a floppy in the drive and format the floppy as a system disk.

if you have anything important on the drive you now want to be the master then back that datra up, do a repair installation and the problem should be resolved