After upgrading to Windows 7, some users saw a new warning message suggesting that they need to replace their laptops' batteries. Some screamed "bug", some shouted "conspiracy", but Microsoft denies that anything's wrong.
In an entry on Microsoft's MSDN blog, Windows division President Steven Sinofsky explains that the warning message is a new feature in Windows 7 and that's why some users are seeing it for the first time on laptops which appeared to run just fine under a different OS:
To the very best of the collective ecosystem knowledge, Windows 7 is correctly warning batteries that are in fact failing and Windows 7 is neither incorrectly reporting on battery status nor in any way whatsoever causing batteries to reach this state. In every case we have been able to identify the battery being reported on was in fact in need of recommended replacement.
He continues to say that this has all the "appearance of Windows 7 'causing' the change in performance, but in reality all Windows 7 did was report what was already the case".