What else are you trying to boot it with? you make no mention.
I know that with Vista the same thing happened to me. In the end I had to run a repair using Vista that then deleted the XP boot partitions
Finally I found an app that I could run from Vista and point to my install DIRs and it wrote the boot screen for me.
I would imagine you're using 7 and XP though so the first thing I would do is run a system repair using the DVD.
The easiest way to dual boot any OS is to install one and then remove the drive. This way nothing gets cross written. Put the second OS on another hard drive and then use your motherboard's boot selection screen (usually F11) to pick what drive you want to boot from.
I'm assuming you have Windows 7 as well and are trying to dual boot 7/XP? In the future, remember to install the older OS first. To fix this, just do a Win7 repair from the Win7 DVD.