Okay, so the Windows XP with SP2 computer boots up, and tells you that HAL.dll is missing or corrupt.

Here's what I recommend you to do.  (No quotes around anything, of course.  Make sure you replace all %variables% with the appropriate path for your machine =).

1. Boot into the recovery console off a Windows XP SP2 CD.  (Hit repair during setup...) Now, we're ready to do some work.
2. First, check to make HAL.dll exists.  Should be in %windows%\system32.  If its not there, copy it over from %windows%\ServicePackFiles\i386 if possible.  You can also do an expand ("expand x:\i386\hal.dl_ %windows%\system32") of the file off a Windows XP CD with SP2.  (Make sure you're using the same CD version as the OS that was installed on the system!).
3. Once that's done, nuke the boot.ini file on the root. Do "attrib -r -h -s -a boot.ini".  Then delete it.
4. Rebuild the MBR.  "bootcfg /list" and then "bootcfg /rebuild"  Load options, you'd want "/fastdetect"
5. Run chkdsk /f
6. Run fixboot
7. Keep in mind - bad hardware is also a potential cause.  The drive you're using may just be on its last legs.