Windows 7 HD wont boot after i swapped with xp HD, then switch back

Hello
I have 2 harddrives in my system that i have been using for a while. both HDs have operating systems, first has vista, and the newer one has Win7 which i use as my primary. when my computer starts, it starts in windows 7.
I unplugged my win7 harddrive plugged in a 3rd harddive alternativly, then booted from a win XP disk to install the OS on the 3rd drive. After my installation of WinXP finished on my 3rd harddrive, i unplugged it, and replugged my Win7 HD. When i tried starting the computer, i received the following error:
Windows could not start because the following file is missing or corrupt:
<Windows root>\System32\ntoskrnl.exe
Please re-install a copy of the above file.

I can still boot from my vista drive if i specify on boot-up, and i can access my win7 drive from windows explorer, however it wont boot due to the error above. does anybody have remedy ideas/steps? or an understanding of what is wrong?
I do not have a win7 start disk, however im in the process of downloading an iso and burning to disk. i do however have an xp start disk if that helps at all...
thank you!!!