Windows XP: xp home- will not boot from hard drive- ERROR LOADING OS

xp home- will not boot from hard drive- ERROR LOADING OS

Computer will boot to windows from hard drive only if emergency floppy is in.
History, when building a music box on an old computer I was moving music files from my main computer to the designated 30 gig music hd. Stupidly, in error, I attached the old 3gig hd from the music box which was set up as master and running W98 This obviously conficted with my main machine master driveand thus my main machine would not boot, after I realized my error I switched the 30 gig in and loaded up my music. The next time I tried to start my main machine I received the error message "error loading os" I changed the bios to boot from the emergency and tried repairing xp, I eventually reinstalled it along with upgrading to sp2. When I try to boot from the hdrive I still get the same error message. Suggestions appreciated. thanks