From: "Rich Gauszka" 

XP and Vista can't coexist nicely?  Must be the MS no-coexist-nicely feature

http://vistasupport.mvps.org/prevent_system_restore_points_being_lost_when_dual
_booting_with_windows_xp.htm

If you are running Windows XP and Windows Vista as a dual boot machine, you
may have noticed that, after booting to Windows XP and then booting back to
Windows Vista, the system restore points in Windows Vista disappear. This,
apparently, is caused by Windows XP's shadow copying and, according to
Microsoft, will not be fixed as it would require extensive alterations to
the code of Windows XP in order to repair the problem.

While there is no real update fix for this problem which, incidentally, can
be extremely annoying, there is a workaround - well, actually two
workarounds, as follows.

Workaround 1
This first workaround relies upon the Bitlocker encryption feature. By
enabling Bitlocker Encryption either via an onboard TPM chip or a removable
USB pen drive you can, effectively, stop Windows XP from removing system
restore points from Windows Vista.

Workaround 2
Another option, which is simpler if you don't want to go down the Bitlocker
Encryption route is to boot into Windows XP and, using a freeware utility
called TweakUi hide the Windows Vista partition from Windows XP. Because XP
cannot see the Windows Vista partition it cannot remove the system restore
points.

--- BBBS/NT v4.01 Flag-5