Assuming you're using a retail version of WinXP, you can try doing a repair of the os. Start up with the WinXP cd, after the drivers finish loading choose the option of installing Windows, accept the agreement, then on the following screen that shows the hard drive or partition that has Windows already installed on it press the r key on your keyboard and follow the on screen instructions.

For more detailed instructions/information you can go here: http://www.michaelstevenstech.com/XPrepairinstall.htm