Microsoft yesterday warned users of Windows XP Service Pack 3 (SP3) that they won't be able to uninstall either the service pack or Internet Explorer 8 (IE8) under some circumstances.

The warning was reminiscent of one Microsoft made in May, when Windows XP SP3 had just been made available for downloading. At the time, the company told users they wouldn't be able to downgrade from IE7 to the older IE6 browser without uninstalling the service pack.

In a post to the IE blog today, Jane Maliouta, a Microsoft program manager, spelled out the newest situation, which affects users who downloaded and installed IE8 Beta 1 prior to updating Windows XP to SP3. If those users then upgrade IE8 to Beta 2, which Microsoft unveiled today, they will be stuck with both IE8 and Windows XP SP3.

A warning dialog will appear to alert users. "If you chose to continue, Windows XP SP3 and IE8 Beta2 will become permanent," Maliouta said. "You will still be able to upgrade to later IE8 builds as they become available, but you won't be able to uninstall them."

She recommended that users instead first uninstall Windows XP SP3, then uninstall IE8 Beta 1; they should then reinstall XP SP3 and follow that by installing IE8 Beta 2.

It's unclear how many users the warning is aimed at. Although users running Windows XP and IE8 Beta 1 could manually download and install Service Pack 3 from Microsoft's site, the company set its Windows Update service so that it didn't offer SP3 to systems with IE8 Beta 1.

Windows XP users who do have the first beta already on their machines will be offered the update to Beta 2 via Windows Update if they have Automatic Updates enabled, Maliouta continued. "A prompt in your Windows task bar will alert you when IE8 Beta 2 is ready for installation," she said.

Windows Vista users, however, will not see IE8 Beta 2 in Windows Update because update apparently cannot sniff out instances of IE8 Beta 1 and uninstall them automatically. Instead, users must remove Beta 1 manually, said Maliouta.