Microsoft has started sending Windows Vista users an update to identify illegal copies of the OS installed with cracks, which the company will disable when it distributes Service Pack 1 (SP1) in two weeks.

The update detects two common cracks used to activate pirated copies of Vista. The company said it would hit Windows Update (WU) within a week.

Users who have left Vista's recommended WU settings alone will receive the update automatically. Others must enable "Automatic Update" within Vista or manually call up "Windows Update" from the "Start" menu.

A document posted to the company's support site spelled out the details. Among other things, it promised that the 3MB update "does not affect the functionality of your operating system."

However, that contradicts what Alex Kochis, senior product manager for Microsoft's Windows Genuine Advantage program, said when he announced the update last week.

There are two cracks. The first, "Grace Timer," extends Vista's activation grace period, while the other, "OEM BIOS," mimics factory-floor activation. If the update finds either, it pops up a notice alerting the user but doesn't disable either crack. Users whose PCs don't have the cracks will not see the pop-up.

Once installed, the update cannot be removed, as it doesn't appear in the "Uninstall or change a program" list in Vista's control panel.

The update is one of the last steps Microsoft's taking before it offers SP1 next month.

That service pack will kill the Grace Timer and OEM BIOS cracks. This move in turn prompts the operating system to start showing messages informing users that they're running a bogus copy and persistently nagging them to pay for a legitimate version.