vista/XP pro dual hard disk

I recently bought a new pc with vista home basic installed on a 160gb sata drive. I had an 80gb sata drive from another pc that I hooked up figuring that I would use vista, but also still be able to access some of the programs I had on XP pro. The OS's were both already installed on the drives, so I thought if I plug in the drive with XP pro, I could see the drive and use it when needed. Since they were both named C:/, all I saw was a C drive with 240gb of space(the 2 combined). I unplugged the XP drive and was hoping I could just change the drive letter, but when I rebooted I got an error stating "NTLDR is missing" hit ctrl+alt+del to restart. Can anyone provide some info on what to do to correct this and possibly dual boot the right way??