You can also set the NTFS permissions such that one OS cant access the drive of the second one.
I thought NTFS permissions are stored on the disk itself. If you set deny permissions in one OS, the same permissions will apply to the other OS too. For example, if you have XP on C: and D:, and if you set deny permissions to D: from the OS installed in C:, you will not be able to even boot the OS installed in D:. The same applies the other way too. I don't think NTFS permissions are the way to go in this case. Best bet is to remove the drive letter from disk management.