Try this: R Repair Installation
Insert windows XP disk, boot the disk, choose R for repair press enter.
Command Prompt choose 1 or C: to repair drive c: press enter.
Command prompt type: fixmbr press enter, answer yes
Command prompt type: fixboot press enter, answer yes
Command prompt type: exit, press enter
If still no luck do a Repair install This way you keep all your files intact but you would need to do updates. here is how to 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, END setup. After the reboot
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.