I'm not sure if you meant that you've used your XP disc to install onto this particular
machine before or just doing dual boots in general. If it's the latter, could it be that
you're installing to a machine that's too modern for XP to install using the drivers it
comes preloaded with (for example, if you have SATA hard drives)? In which case, your
solution is to create a new install disk by 'slip streaming' the required drivers which
you can achieve using a free program called nlite (it's easier than it sounds).
By the way, it's tricky to get XP working elegantly in a dual boot with Windows 7 unless
XP is installed first. You'll encounter headaches like the system drive not being C: and
the Windows 7 drive still being visible with no way of hiding it etc etc. I've just been
through that particular brand of hell myself.