If XP won't start it may be due to a damaged boot sector or a missing or corrupt ntldr or ntdetect.com files.
To replace damaged ntldr and ntdetect.com you can copy fresh files from the XP CD using the COPY command:
1. Boot with the XP CD and enter the Recovery Console.
2. At the Command Prompt type the following (assume "E" is your CD-Rom drive letter) allowing the files to overwrite the old files:
a) COPY X:i386NTLDR C:
b) COPY X:i386NTDETECT.COM C:
To repair a damaged Boot Sector at the command prompt type FIXBOOT and press Enter. Then answer "Y" and reboot ur xp !
If the above doesn't work then follow the below :
1. Insert the XP Boot CD in CDROM Drive
2. Press Enter to start the Windows Setup.
3. To setup Windows XP now and Repair Install
4. Press ENTER. do not choose "To repair a Windows XP installation using the Recovery Console, press R", (you Do Not want to load Recovery Console).
NOTE: Do not choose "To repair a Windows XP installation using the Recovery Console, press R" option....
5. Accept the License Agreement
6. Select the XP installation you want to repair from the list and press R to start the repair. If Repair is not one of the options, END setup.
7. After the reboot Setup will copy the necessary files to the hard drive and reboot.
CAUTION: Do not press any key to boot from CD when the message appears. (cause xp checks RAM in this stage)
Setup will continue as if it were doing a clean install, but your applications and settings will remain intact.
Even, this not works, then format xp and perform clean install, if u don't have necessary files !