Unfortunately, you can't boot a hard drive like that every time depending on hardware differences. Unless you want another computer setup, I would get an external enclosure or dock for IDE hard drives in order to get the data off. Otherwise, if you reinstall, you will lose that data. You may be able to do a repair install of windows xp, but if you need the data, I'd get that off first using the above method before trying anything.
My old motherboard crashed so I replaced with a new computer using Windows 7. Old system had IDE harddrives and some of the data stored had not been backed up. My plan was to install old drive as a backup in the new computer, but it accepts only SATA hard drives. My next plan was to install the old hard drive in an older computer as the C: Drive, then burn CDs and transfer data that way. I had all the necessary application software so this seemed simple.
Old computer had Millenium SE as its original OS and replacement harddrive had XP Pro.
I am now getting a "NTLRD is missing...Press any key" message and cannot complete the boot sequence. Old machine is an HP Pavilion XE 738.
Is this old machine just too wimpy for the XP OS, or, if my plan should work, what should I do? I have the XP installation disk.
Thanks.

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