An 'Important Update' arrived last night via Windows Update on my Vista PC. I'm running the RC1 of Vista Service Pack 1, and since installing that I've not received any other 'Important' updates so I investigated further. The update is described in the MS Knowledge Base article 940510 and is an update that's intended to weed out illegal or cracked installations of Vista.
If the updates detects exploits that indicate you have a counterfeit or hacked copy of Vista, it will warn you and point you to a Microsoft website that will enable you to remove the exploits. If you don't want to do this Windows may disable the exploits and then ask you to use a valid product key to activate Windows.
According to a discussion on the MS forums, the update is actually a one-time check for two of the most common Vista hacks, the 'OEM drive activation' and 'grace timer activation' exploits, which are described in another KB article.
Microsoft, Windows Vista, Windows Update, Hacher, Anti-hacker, Patch, Security Patch, Release