In a quick turnabout, Microsoft Corp. made the newest tweak to Windows Vista Service Pack 1 (SP1) available to the public on Friday.

Just two days earlier, the new version, dubbed Windows Vista SP1 RC Refresh, had been handed out to a group of about 15,000 testers who had been working with the service pack for several months. At the time, Microsoft said the refresh was "not available for public download."

Friday, it changed its mind and posted instructions on its Web site for downloading and installing the new code using the Windows Update service.

According to the Windows Vista Service Pack 1 RC Refresh Public Availability Program, users must uninstall Vista SP1 Release Candidate -- the earlier version offered to the general public a month ago -- before attempting to download and install the refresh.

The refresh requires the same time-consuming, multiple-reboot process used by Vista SP1 RC in December. Also, users who have uninstalled that version must wait an hour before beginning the laborious update. "The installer service needs to clean up and complete the uninstall prior to installing the RC," said instructions posted on the Web. "Failing to do this can result in installation errors when installing the RC version."

Three prerequisite updates are also required before SP1 can be installed. Windows Update feeds them to the PC prior to downloading SP1, with a reboot after each. One of the prerequisites is a patch Microsoft mistakenly sent to all Vista users' PCs last week when it meant to send it only to machines running Vista Enterprise or Vista Ultimate.

The company, which has slated Vista SP1 for final delivery this quarter, said as recently as Thursday that the update remains on track.