I believe the Windows XP from the old hard drive detected that the new hardware configuration is different from the one it was setup in, that's why it could not boot up.
You may need to repair Windows XP. But since you want to retain your data, I suggest that you put the hard drive back in the old pc. Partition it into drive C: and drive D: (http://www.click-now.net/html/PartitionMagic.htm
Then transfer My Documents (which contains all your data, I hope!) to the newly-partitioned drive D.
Now transfer the hard drive to your barebone system and do a Windows repair or fresh install on drive C:.
Hope this helps.