If I have Windows XP installed on a hard drive configured as Master, could I change that hard drive to slave? Or would I have to reinstall Windows once I changed it to Slave? The reason I'm asking is, I'm adding a second hard drive with a SATA to IDE connector that automatically assigns the SATA drive to Master. After I add this drive, I'm hoping I can reconnect my Windows drive (which is currently a Master) as a slave.
I want to be sure you understand the differences among Master, Slave, and Boot Drive.
Master and Slave are ONLY terms used in connection with IDE (aka PATA) hard drive systems. On any IDE port you can have up to TWO devices, and so each needs its own unique identifier. That system is accomplished with jumpers on pins on the device, and you set each device to either Master or Slave. Each IDE port MUST have a Master device to operate, and MAY have a Slave device also sharing the port. For this purpose the 80-conductor ribbon cable for data has three connectors - one (Blue) for the mobo port, one (Black) at the opposite end for the Master, and one (Gray) in the middle for the Slave.
SATA ports can only have ONE device per port, so there is no such thing as a Master or Slave for any SATA device.
In any computer, there must be one device at least that serves as the Boot Device, from which the BIOS can load and run an Operating system. Years ago by default this was the Master device on the Primary IDE Port, so a lot of people got into the habit of thinking Master device is Boot Device. That is NOT the case, and has not been for some time.
In current BIOS's you set the Boot Device yourself. In fact, normally there is a Boot Priority Sequence you set up. You can set two or three or more devices to try to boot from, in the order specified. Most people set them to try the optical drive first, then (if that fails to provide a bootable OS) go to a specific hard drive unit. If your machine has more than one HDD, you set it to the particular one that actually does contain the OS. (In my case, the order is: floppy drive 1st, optical unit #1 (I have two) 2nd, HDD #0 (also two in machine) 3rd.)
When this is set up, most OS's will by default set the drive they just booted from as the C: drive, and then assign letter names to other devices in the system. The assignments are usually stored somewhere in a configuration file on that C: drive, so they don't change from time to time unless some change it forced.
So now to your question. If you are using the terms correctly and you have Win XP installed on your Master drive on an IDE port and simply want to make that unit take on the Slave position (with some other device as the port Master), yes, you can do that. You will have to change the BIOS Boot Priority Sequence so that it points to that HDD again (now the Primary Slave, rather then the Primary Master). After booting it will still be called the C: drive.
On the other hand, if you intend to install some other OS on another HDD unit as a boot device and NOT boot from the HDD that has Win XP installed, you most certainly can do that. Once you've completed the new arrangement, that older drive will no longer be the C: drive since you don't boot from it - it will get a new letter name under the new OS. However, it still will be fully usable as a drive.
To install a new OS on a new HDD unit, I suggest a few things. IF your intent is to create a dual-boot system in which you have to chose at each boot-up whether to boot from the new OS or from the old Win XP, you can do that. If the new OS is a newer version of Windows, it will do most of the work for you. You would connect the new HDD in your system, then enter the BIOS Setup screens and set the Boot Priority Sequence to the optical drive first, the new HDD second, and the old HDD third. Place the new OS's Install Disk in the optical drive. Save and Exit your BIOS Setup screens and the machine will boot from the Install disk like any normal Install process. If it's a new Windows you are installing, early on it will detect the existence of Win XP on an HDD and ask whether you want to create a Dual Boot system. If you say yes it will take care of setting that up, then proceed with the Install of the new OS.
On the other hand, if you do NOT want a Dual Boot system, I suggest you connect the new HDD but DISconnect the old HDD for now. Set the Boot Priority Sequence to Optical, new HDD, and NO other choices, put your OS Install disk in the optical drive, Save and Exit. Complete all the Installation on the new drive. When it's done, shut down and reconnect the old HDD. Reboot and Windows will simply recognize the new piece of hardware added and make it available in My Computer. The reason for this is to defeat an interesting safety feature of Vista and Win 7 (odd idea!) When those newer Windows version Install, if they find a second HDD available, they will place a semi-secret copy of their important files on that disk before installing to the new target disk. The idea is that, at any subsequent boot-up, if something wrong is detected it can use the backup copy on the second HDD to restore to the proper boot disk and fix itself. BUT this also means that, once this is set up, the system cannot boot up at all unless that second disk is present. So you cannot remove that second disk after the fact!