If you mistaken deleted NTDETECT.COM, you won't be able to boot up the computer because it is used during the hardware recognition when boot up.
To solve this problem:
1. Insert Windows XP installation CD (bootable).
2. When system prompted to press any key to boot from CD, press Enter (make sure that your bios is set to boot from CD first).
3. In Windows XP setup, press "R" key to repair windows in the recovery console.
4. Log in to the windows installation by pressing "1" key and enter.
5. When prompted for administrator password, enter the password you set.
6. Copy 2 file from the CD-ROM to the root directory of your primary hard disk (For example: e is the drive for CD-ROM)
copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\
7. After finished copy both the files, remove the Windows XP CD and restart computer.