Microsoft has released a new update for Windows Vista and Longhorn Server

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. After you install this item, you may have to restart your computer.

Update for Windows Vista Beta 2 and Windows Codename Longhorn Server Beta 2 (KB919946)