how do I reinstall <windows>\system32\hal.dll. on HP pavilion PC running windows XP?
This PC will not start windows.
You need your XP CD (or a borrowed one, preferably Professional) and you boot from that and go through the initial motions of reinstalling, up the point where you press Function 8 to accept the terms of the EULA and after that, select R for Repair. When prompted which installation you want to repair, type the number 1 and hit Enter. At the Command Prompt, you need to run the System File Checker so that any important missing files can be traced and replaced so type sfc /scannow (including the space) and hit Enter. Restart after that finishes and see if the system boots properly - just don't hit the "Press any key to boot from CD" that time.