grandson and I share computer.want to add another drive but want to try to set up one drive for him to log onto and other drive for me

Welcome to PCHF voltron
Well then your talking about dual booting.For this im not sure but if your going to use Windows Xp you might need to buy another copy.But after you do you can insert the Xp setup disk or whatever operating system you want and select the other harddrives Letter.If you want i can explain it i depth.

Why not just set a password on your account... and encrypt any files you don't want him to get at...? Really no need for separate drives. It is important that you do NOT use this on Windows system files-- personal stuff only.
Unless he's a "cracker"-- in which case you won't keep him out anyway. Someone who knows his stuff can't be kept out of a machine he has physical access to.
This is the whole purpose for user accounts-- in XP.
You could make his account a 'limited user' account-- he can't install stuff that way.

How to create and configure user accounts in Windows XP

How to make files and folders private in Windows XP so that only you have access to them