In Windows operating system .DLL files are the most important files. If  any files are corrupted or missing then the operating system cannot function properly. After logging to the system, if you are facing any error that .DLL files are corrupted or missing then you can copy from another system. But if you get this error while booting the OS, then you need to follow the steps below

Error:When you power on the cpu, you are getting the error as,

“Windows could not start because the following file is missing or corrupt:<Windows root>\system32\hal.dll.
Please re-install a copy of the above file.”

Solution:
Step 1 : Power on the system and insert the windows XP cd.
Step 2 : select the startup device as CD-ROM and boot the system.
Step 3 : Press any key to boot from the CD.
Step 4 : After the setup files are finished loading, press R to repair using Recovery Console.
Step 5 : In the recovery console, select the installation to log on to (usually number 1).
Step 6 : Type 1 and then press ENTER.
Step 7 : Enter the administrator password when it prompts, and then press ENTER.
Step 8 : At the recovery console command prompt, type the following command,
Step 9 : Type expand e:\i386\hal.dl_ c:\windows\system32 (this command will expand the content of hal.dl_ into system32 folder)
Step 10: C: drive contains system OS files and E: drive indicates CD-ROM.
Step 11: Goto c: drive and type bootCfg /rebuild
Step 12: It will take time to rebuild and then restart the machine.

By the same way we can copy the .DLL files from XP cd to system32 folder.