|The only way you will get that setup to work seemlessly is to disconnect the existing hard drive and install WinXP on the spare drive. Then disconnect that drive and use the original.|
Otherwise you will create a dual boot situation that will no longer work if the original craps out.
Better to use imaging software to maintain current images of your existing drive. If it ever fails you can use those image disks to restore everything to a new drive and all will be as it was.
Going your way will require you to constantly update the spare drive. If you do attempt to keep both connected then you will need to learn how to repair the master boot record in case you need to disconnect your original drive for any reason.
If you insist on trying your way then just boot to your original WinXP and then insert your WinXP CD and install to the second drive. You will then get a choice of which to boot to at start up.