If you have the Windows XP CD-ROM available, you must run it and restore the OS kernel reinstalling it (Note: I don't mean that you reformatting the HDD, only reinstall Windows XP to restore the kernel)
try it and answer me back!!!!
If you don't know how to do this, you must follow the next steeps:
1. Boot up the machine with the Widnows XP CD-ROM inside, and press any key to start from the cd...
2. Select in the blue screen menu the "Install Windows XP" option by press the Enter key.
3. When the Installation interface shows you the option that you have a Windows XP OS installed, then press R to repair the Installed WXP.
4. When all the process finish, you could enter to WXP normally, and you will note that all the programs, folder and files will remain in your pc.