I think what you’ve done is either swapped the drives around on the IDE cable or installed the second OS isolation of the 1st which is why the system doesn’t see the original. When you fitted the new drive & initially re-installed XP, did you leave the original drive where it was or move it onto the slave IDE cable connector, effectively making it drive D? If so, that’s why the system no longer sees the original XP OS as there is no information that it exists in the primary boot sector on what is now your master drive C; if you want both OS to appear in the boot menu, you must install the second OS with the first still visible in the primary boot sector or it won’t work.
You will have to re-install XP onto what is now your slave drive (D). You don’t have to install into a logical partition on your master C drive. Stick the OS disc in, boot & initiate setup; you will be asked if you want to reinstall (reformatting the existing C drive) or do a new OS install. Chose new installation & you will then be asked where you want to install the new OS & both drives should appear in a menu at the bottom of the screen; chose the D drive & setup will then re-format & install a new XP OS on that drive. After setup has completed, you will see a boot menu showing both OS allowing you to chose but, unfortunately, XP will be the default OS as it picks the latest install as default; I have no idea if you can change it.
I have used “Boot Magic” (a boot loader) in the past but it wasn’t without problems & I don’t think it works with the new file systems anyway; better to use Windows own boot manager.