No it wouldn't be a dual boot.
To have a dual boot system you have to install 2 different operating systems into the same computer on separate partitions or drives. Install oldest OS first. It is possible to have 95, 98, 2000, xp & linux all booting on the same system. Just takes a lot of work. (Helps if you enjoy that kind of tinkering)
There are a couple of ways to do what you want.
First off when you build a new computer you really want to start with a clean (preferably new) formated, partitioned hard drive. Then install windows, then motherboard drivers, directx, usb drivers etc. Then video, sound dvd what have you.
Once all that is done, you have the choice of
A hook the 2nd "old" drive up as a slave, either manually copy, use hd utility's to "disk copy" or otherwise copy the info on the "old" drive to a clean partition on the new drive.
B Hook the "Old" drive up as slave & leave it there.
Either way I'm afraid your looking at a new copy of XP, unless your a pirate & are willing to cheat, lie or otherwise bend the rules.