Boot with Safe Mode, Uninstall any software you just installed which may cost trouble, or you can use Restore Point.
If you cannot boot into Safe Mode. You need to do repair, boot PC up with XP setup CD and get into Recovery Console, run Fixboot.
NOTE: DO NOT do anything you are not sure if you have not backup your files. If you have some important files on PC, it is better to ask a PC guru to help you to sort things out.