Which operating system ?
some operating systems have part of there operating system stored in partitions which are protected other operating systems like Vista have partitions which have there recovery back up and also are protected
Sometimes there's no better way to repair a damaged Windows XP install than to start from scratch and reinstall the whole thing. It's a drastic move, but it is almost guaranteed to work. (Only serious hardware issues or incorrectly installed files can cause errors in this case.) If all other options have been exhausted, and your computer still won't function correctly, fear not: After a reinstall it should be running as good as new.
Locate an XP install disk, preferably the one that came with the computer, and any other software packages and drivers. Most of the time these will come packaged with the computer at the time of purchase. To save your data, copy information in your "My Documents" folder and any other important data you might have stored on the hard drive to a flash drive, CD or an external hard drive. Insert the XP install disk into the computer's CD drive and reboot the computer. The computer should automatically boot from the disk, loading the blue Windows XP Setup screen. On some computers, you may need to press a button when prompted to boot from the disk. After the Windows XP Setup has finished loading files, press "Enter" as prompted on the screen to set up Windows XP, and then accept the Licensing Agreement on the next screen by pressing "F8." Navigate the partition menu to find the partition on which Windows XP had previously been installed (likely, there will only be one) and press "D" to delete it. Verify that you want to delete the partition on the screens that follow. Set up a new partition, press "C," set the desired for the partition and press "Enter" to create it. The new partition will now be listed on the previous partition menu. Select the newly partitioned space, and press "Enter" to set up Windows XP on it. The Windows XP Setup program will format the new partition and continue with the graphical user interface portion of the installation process after the computer reboots. Navigate through the set-up menus that follow. The setup program will reboot the computer and load Windows after it has finished installing files. Reinstall any drivers and software that may previously have been installed on the computer by inserting their disks and going through their installation processes. Download Windows updates from Microsoft's website. (See Link Below.) http://www.microsoft.com/windows/downloads/windowsupdate/default.mspx