Some Windows 7 users have reported that their PCs started to freeze or randomly display the infamous "Blue screen of death" after applying a January update from Microsoft.

Microsoft today said it doesn't consider the problem a "major issue," but acknowledged it was investigating.

As first noticed by Ars Technica, a short thread on Microsoft's Windows 7 support forum discusses the update, which Microsoft issued two weeks ago, on 25 January.

The update is designed for both the 32- and 64-bit versions of Windows 7, as well as Windows Server 2008 R2.

"I just installed this update and my system hangs/freeze[es] at the Windows bootup screen," said a user identified as "Kamonk2" on the support thread Jan. 27.

"I had the same issue on my brand new laptop," echoed "MacPCGraphics" on 29 January. "It worked fine until the Auto Update kicked in and installed an update. Started hanging and freezing after that."

Another user offered an even more detailed description of the update's impact.

"Windows Update installs the KB977074 update but after rebooting, my system hangs. To be more specific, the computer boots up and begins to start Win7...I then get a message to say that 'Windows is installing updates...Do not turn off your computer...0 (zero) %,'" said "dunghoaxinh" on Jan. 30. "Unfortunately, it just stays on '0%' and the system seems to hang here. The only way to get around this is by cold reboot and system restore to before the update."

Several of the users affected by the update reported that they regained control of the machine by uninstalling the 25 January stability and reliability update. "I also removed KB977074, which fixed my problems," said "bob_az" on the thread a week ago.

A Microsoft-employed forum moderator had other advice. "For the people who installed [the update but] cannot start the computer normally, it is better to wait for the next stability and reliability update," said Arthur Li on 1 February. "Since there are thousands of different hardware and software configurations, it is hard for Microsoft to test the updates on all the different hardware and software configurations."

Today, Microsoft downplayed the problem and denied that there's any evidence the 25 January update is ot blame. "We have not seen customer problems with KB977074 as a major issue within our customer support channels," a company spokeswoman said in an e-mail. "We are aware that some customers are having issues and are working to identify the cause. At this point, there is no indication that this specific update is the cause of install, stability or reliability issues with Windows 7."

Windows upgrades and updates occasionally run amok. Last autumn, for example, Microsoft's support forum lit up with complaints from customers whose machines were trapped in endless reboots after an attempt to upgrade from Windows Vista to the then-brand-new Windows 7.