A lot of new motherboards also have a "Boot Menu" during post, press a certain key (ex: on my laptop it's ESC) and a menu will popup asking what you'd like to boot off of HD1, HD2, CDROM, Network, etc. But the way you're phrasing it, sounds like you don't even want to be able to access one HD while running windows off the other one (IE: the SATA drive not being accessable form Windows when running off the IDE drive), in which case, there's a thing kinda like a KVM switch for HDs; you turn the computer off, press a button, reboot and boot from the other HD. I don't know if they work with SATA, though. Removable drive cages are always an option too. Just physically take the drive out when you don't want it to be working.
Edit to ask: Why do you want to boot from your old HD? If all your data is accessable from the old one, why not just leave it? Also, do you have the option of putting together your old computer and networking it to the new one? That way you can boot off the old HD by simply turning on your second rig.