So i am guessing that you dont have the Windows XP CD. That would allow you to run a repair on the partition.
1. Call HP and request a XP CD.
2. If you have your serial. Download XP. Use YOUR SERIAL and do a repair.
Anything else will not really help. The hal.dll is a needed file and can only be relaced by a repair or format-reinstall. Just adding the .dll will not fix the issue. I know i have tried before.