Microsoft has warned users updating to Windows XP SP3 (Service Pack 3) that they won't be able to downgrade from Internet Explorer 7 to the older IE6 without uninstalling the service pack.

The warning first appeared in a post Monday to a company blog written by the IE development team. Microsoft released Windows XP SP3 to Windows Update as an optional download Tuesday.

"If you choose to install XP SP3, Internet Explorer 7 will remain on your system after the install is complete," said Jane Maliouta, an IE program manager, in the blog entry. "Your preferences will be retained. However, you will no longer be able to uninstall IE7. If you go to Control Panel, Add/Remove Programs, the Remove option will be grayed out."

The inability to downgrade to IE6 after installing XP SP3 was by design, said Maliouta, because the service pack includes newer versions of the old browser's files. If Microsoft had allowed users to revert back to the pre-SP3 version of IE6 -- the one saved on users' PCs when they upgraded to IE7, and until now what was used to back out of the newer browser -- Windows would have ended up in a "mixed file state," Maliouta said.

"This state is not supported and is very bug prone. To ensure a reliable user experience, we prevent this broken state by disabling the ability to uninstall Internet Explorer 7," she said.

Users who want to retain the ability to downgrade from IE7 to IE6 should uninstall the former before upgrading to XP SP3. Once Windows XP has been updated to SP3, users can then install IE7. That process allows for reverting to IE6 in the future.

"The restriction on uninstalling only applies to when you install a Windows Service Pack release on top of a standalone IE release," Maliouta said.

If Windows XP SP3 has already been installed, the only way to return to IE6 is to first uninstall the service pack. At that point, IE6 can be restored on a PC that's been updated to IE7.

Microsoft released IE7 in October 2006; it was the first major update to Internet Explorer since August 2001, when IE6 went final.

The newer browser has not been able to usurp IE6, particularly in businesses, where it remains Microsoft's most popular browser. According to a survey released in late March by Forrester Research, only 30 percent of corporate Internet Explorer users had switched to IE7 by the end of 2007. IE6 accounted for nearly all the remaining 70 percent.

Maliouta also outlined how Windows XP SP3 upgrades affect in-place copies of IE6 and IE7; in both cases, she said, the currently installed browser remains undisturbed by the update.

However, users who have installed IE8 Beta 1 -- a preview of its newest browser that hit the streets two months ago -- will not be offered Windows XP SP3, according to Maliouta, again because of possible instability problems.

"We strongly recommend uninstalling IE8 Beta 1 prior to upgrading to Windows XP SP3 to eliminate any deployment issues," she said, "and install IE8 Beta 1 after XP SP3 is on your machine."