After turning on my laptop, I accidentally doubleclicked (rather than singleclicked) the Mathematica icon (on the taskbar) and got two open sessions.
I then closed each, returned to the desktop, and then singleclicked the icon. Mathematica tried to activate but couldn't (a Microsoft error box appeared).
Upon inspecting ProgramFiles\WolframResearch\Mathematica\6.0\SystemFiles\FrontEnd\Stylesheets, I found that StyleSheetFormatting.nb was 0 bytes. After restoring this file from a backup disk, everything works fine.
Question: Why did doubleclicking the icon zap the above notebook? Thankx.
Bruce