When you start Windows XP, you receive one of the following:
<b>STOP: C0000221 unknown hard error
Path \ File_name
STOP: 0xC0000221 unknown hard error
Path \ File_name</b>
When Windows XP starts, it checks all DLLs and drivers for integrity, except the ones needed by the system loader to start the computer. If any of these files are corrupt, you will receive one of the preceding STOP messages. The actual message format depends on how much of Windows XP was loaded prior to the error.

To fix the problem, you need to replace the corrupted file, either from the Recovery Console, a Repair, or re-install.