u can repair ur windows XP OS without loosing any setting or data,
if u have any missing OS file or any errors, repairing will fix it.
Reboot the computer with first boot device as CD-Rom
Place the Win XP cd in CD-Rom
At the prompt "Press any key to boot from CD" press any key Setup starts-
Here u also see a Repair option but dont use it-
Press Enter to continue setup-
Press F8 to accept License Agreement
Now setup will search for previously installed Operating systems, a list will be displayed
(Note: if u have windows 98 it wont be displayed in the list)
Select the Operating system which ur repairing, like "C:\Microsoft Windows XP Professional" Press 'R' to repair Setup startup copying files and Restarts
(Note: if the file copying process is not done and the system restarts u have to again follow the same as above by booting from CD-Rom),
after file copying is done system restarts
Now u have to boot from HDD not from CD,
At the prompt "Press any key to boot from CD" dont press any key,
setup will continue U need ur CD key
Warning: If u have downloaded Service Pack 2 and installed it then u have to again download and install it after repair If u have ur Service Pack 2 inbuit in the XP CD then no need to worry, it will get installed along with Os