Updates that Microsoft began feeding Windows Vista users last week to prep PCs for next month's release of Service Pack 1 (SP1) have crippled some machines, according to messages posted to the company's support site.

Microsoft said it is investigating the reports.

Last week, Microsoft started sending Vista users two final prerequisite updates that are required before SP1 can be installed in March. The updates to the operating system's install components were delivered via Windows Updates, which automatically downloaded and installed them on the majority of Vista machines.

Users quickly started squawking.

In most cases, they reported that the final update hung while displaying the message "Configuring Updates Step 3 of 3 -- 0% Complete," which was followed by a reboot of the PC. Which was followed by another reboot, and another. "[It] reboots ad infinitum," said Frank Melk on the Microsoft support newsgroup. A smaller number reported a different problem: After the update, their computer refused to boot normally.

Trying to boot into Safe Mode did no good, users said; the reboot loop cranked up then as well. "I am unsure as to what to do, because entering Safe Mode gives the same screen," Melk said. "Furthermore, I have no restore points saved, so going back to a known previous good config is no good either!"

Melk's mention of restore points referred to Windows Vista's System Restore, a tool that periodically takes a "snapshot" of the PC. Also called restore points, they can be called up to return the machine to its condition at the time the snapshot was taken. Some users who posted messages to the same newsgroup said that they had managed to regain control of the computer by booting from their Vista install DVD and selecting the "Restore from a previous restore point" option.

"The first two restore points available to me failed," noted another user, pegged as phazedoubt. "I had to go back three days before I found one that worked."

Others said they had been in touch with Microsoft support representatives -- the company offers free support to consumers on all update issues through a toll-free number or e-mail -- and claimed that they had been told to boot from their Vista media and choose "Run Startup Repair."

"Apparently, so Microsoft says, my machine was restarted thinking it had downloaded an update, but really the update hadn't been downloaded," said user bicksbah on the support newsgroup. "So, upon reboot, it couldn't find the update and Vista kept trying to install it endlessly."