You won't have any restore points in Vista after booting into Windows XP. See
this article for more info:
No restore points are available when you use Windows Vista or Windows Server
2008 in a dual-boot configuration together with an earlier Windows operating
system
http://support.microsoft.com/kb/926185
There are a few work-around scenarios mentioned in this article. None of which
simply advise turning off the System Restore feature in Windows XP, which seems
to be a logical thing to do since I use Acronis True Image and ERUNT for
backups. I'm going to try doing that (disable System restore) on my friend's
laptop, which I'm still working on to get to dual-boot properly. The major
problems I'm having with his system is obtaining the proper device drivers (This
is an HP laptop with Vista installed). FWIW, I really don't have much hope for
this experiment.