1. Insert boot disk of Windows XP and let it starts from installation disk. (make boot sequence as CDROM from BIOS setup)
2. After some time, when the screen comes with Repair a Windows XP Installation, select it
3. Type administrator password (if any) if not just press enter
4. Now type fixmbr at the command prompt
5. When prompt comes, again type fixboot C: (If not, select the drive letter where u installed earlier)
6. Now type Exit and restart computer
NOTE: If this is not comfortable to u, u may go for full repair which is conducted by XP. i.e. second option. Follow no. 1 and bypass Repair a Windows XP Installation option, and follow on-screen instructions, and choose for Repair. This for GUI repairing method, which performs by XP.... Don't worry in this process all settings will be intact and the files r also okay. Remember, I am asking for only REPAIR, not for format or partition. So be careful when performing the task.