There is two or three things you could try, first being a repair install of the operating system, which does not erase your personal files.
See this XP forum, sixth post from the top, for the directions. you will need an XP disk.
If that fails, you can take the drive out of the computer and install it as a second drive on a working computer. You can then make a backup of your personal files, which are still on the drive.
Then reformat the bad drive and install a fresh operating system.
OR you can have a computer shop recover your files, as above, and then re-install the operating system.