Ugh I dont know!! Maybe becuse you choose XP and then used Win 7 installation disk when Win 7 was on another HDD, Master Boot Record got messed up somehow? I am not 100% sure but I think when you have two HDDs, each of them has MBR. Can you try sticking the 7 installation disk, boot FROM THE DISK (by setting your CD/DVD drive as the first boot device), chose "repair" --> Windows start up error. Then reboot to see if that fixes the problem?? I think the only way to shake things and get something going from here is to boot from a disk.
Also, you can removed the XP drive once, and fix Win7 boot first and then add back the XP drive, just to make it simple. If XP doesn't boot after that, you could try using XP disk to fix XP's boot error.
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HTPC + Media Server: http://www.eggxpert.com/forums/permalink/5855/836221/ShowThread.aspx#836221