Microsoft have started releasing updates during the Windows Vista/Longhorn beta, this one is of particular interest due to the fact that many people -including myself, have a hyper-threaded system with Windows Vista Beta 2 installed on it (not my primary system, in case you were wondering).
Update for Windows Vista Beta 2 and Windows Codename Longhorn Server Beta 2 (KB919946)
Install this update to fix an issue where in the presence of Internet Protocol security (IPsec) or Windows Firewall, a race condition can be hit on multi-processor (or hyper-threaded) systems causing memory corruption, leading to a system crash.
It is unknown if this affects the recently released build 5456, fiile date is 27th so it probably does.