The first thing you will need is your Windows XP Installation Disk. If you don't have this, the recovery disk that came with your computer might also do the trick.

Boot from your Windows XP original CD by putting it in the CD drive and restarting your computer. Then go to the recovery console (you should be prompted to press R to go to the recovery console). You should then get a command prompt.

Type: (and press Enter after each line)

C:
cd\
attrib -c ntldr

Then restart your computer again. Hopefully that will do the trick.

However, if it still doesn't work, reboot with the CD again, go back into the recovery console, and type :

fixmbr
fixboot

Then restart once more.

If you have an HP computer with a recovery disk instead of a XP install disk, then you should read these instructions for how to get into the recovery console in order to fix the ntldr error.