Days after the Windows 7 Release Candidate becomes available, Microsoft has to send out a critical patch. Now it only affects the 32 bit version, and honestly, I think that Microsoft is stupid to make a 32 bit version. All the processors on new machines can support 64 bit, so why not use it.
Anyway, the issue comes in with how ACL lists are handled. You can read more about it here.
Also, on Windows 7 RC patches, in the next week there will be a bunch of fake patches sent out by Microsoft that do nothing. Well, nothing but test the updating system, which is somewhat important. Make sure to use these for testing the system.