Dualboot XP Vista and Restore Points – have your cake and eat it
When you configure a dualboot Vista/XP or Vista/2003 and you’re using separate partitions, by default both operating systems try to store undo information (restore points).
If you then start the older OS, it ‘ll delete the restore points from the newer OS.
http://support.microsoft.com/kb/926185 describes two separate methods to work around this behaviour, either using a registry entry blocking Restore point creation or by using Bitlocker.