On releasing the service pack, Microsoft said one of its main benefits was that it would make it easier to interoperate with Microsoft's latest operating system, Vista, and its latest productivity suite, Office 2007. The older file formats that are now blocked are in decreasing day-to-day use, but the blocking of them will make retrieval of archived material more difficult.
The changes were revealed in a Microsoft support document, which was uploaded to its site in December. Users were given no warning of the effects when they downloaded SP3.
In the support document, Microsoft said SP3 blocked access to those formats because they were less secure than newer versions. "By default, these file formats are blocked because they are less secure. They may pose a risk to you," it said.
Microsoft released details of a work-around to restore access. The work-around requires changes to the registry, which could render a PC unusable if carried out incorrectly.
The work-around was branded by one critic on tech Web site Slashdot as "mind-bogglingly complex."
Other users responded negatively to the change. A system administrator at a U.K. university, who asked not to be named, called it "a money-making exercise," adding that it would cause a problem to the central IT resource not to have access to some older file formats but that the effect would be greater on other less "progressive" departments within the university.
Microsoft could offer no comment at the time of writing on why it had blocked access to the file formats.
Richard Thurston of ZDNet UK reported from London.