You can do this:
XP Repair install
Boot the computer using the XP CD. You may need to change the boot order in the system BIOS so the CD boots before the hard drive. Check your system documentation for steps to access the BIOS and change the boot order.
When you see the "Welcome To Setup" screen, you will see the options below
This portion of the Setup program prepares Microsoft
Windows XP to run on your computer:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
Press Enter to start the Windows Setup. do not choose "To repair a Windows XP installation using the Recovery Console, press R", (you Do Not want to load Recovery Console). I repeat, do not choose "To repair a Windows XP installation using the Recovery Console, press R".
Accept the License Agreement and Windows will search for existing Windows installations.
Select the XP installation you want to repair from the list and press R to start the repair. If Repair is not one of the options, read this Warning#2!
Setup will copy the necessary files to the hard drive and reboot. Do not press any key to boot from CD when the message appears. Setup will continue as if it were doing a clean install, but your applications and settings will remain intact.
or
1. Start your computer.
2. Insert the Windows XP CD in your computer's CD-ROM or DVD-ROM drive.
3. On the Welcome to Microsoft Windows XP page, click Install Windows XP.
4. On the Welcome to Windows Setup page, click Upgrade (Recommended) in the Installation Type box (if it is not already selected), and then click Next.
5. On the License Agreement page, click I accept this agreement, and then click Next.
6. On the Your Product Key page, type the 25-character product key in the Product key boxes, and then click Next.
7. On the Get Updated Setup Files page, select the option that you want, and then click Next.
8. Follow the instructions that appear on the screen to reinstall Windows XP.