Problem with bootable WinXP hard drive (enclosure)

Hi,
My laptop recently died (motherboard likely). I took the hard drive out and put it into an enclosure. I can access the files just fine when I use the drive as a USB hard drive on another system, does not seem corrupted, ran chkdsk /f from dos, error checking from windows, disk cleanup, disk defrag, everything seems fine

BUT, when I try to use it as a bootable WindowsXP (pro) drive by selecting "boot from USB device" on my other WindowsXP system, I have no luck. What happens is this:

Windows failed to start. A recent hardware or software change may be the cause.... options: 1) try to repair, 2) start windows normally;

whichever option I choose leads to the same result... I see the green bars sliding across the screen as windows appear to load, but then the green bars get STUCK, and then I get the BLUE SCREEN>>>

"A problem has been detected and windows shut down to prevent damaage. Check for viruses. Remove any newly installed har drives or hard drive controllers. Check your hard drive to make sure properly configured and terminated. Run chkdsk /f to check for hard drive corruption, then restart your computer. Technical information: **STOP 0x0000007B (0X80699B0, 0xC0000034, 0X00000000, 0X00000000)
"


ANY hints ??

Thanks so much!!