This has been reported to be happening after fresh install of Windows 7 SP 1. But, it happened to me on an old netbook with Windows installed three years a go or more as well. There were also reports, that fix shown in this article can help for Windows Vista SP2 not being able to install any new updates. I’m also pretty sure, that certain Microsoft downloads (even those offline) are affected by this problem, i.e. until you fix it, you won’t be able to install .NET Framework (for example) on freshly installed Windows 7 SP1.
Anyway, if at any point of time on any Windows you’ll notice, that Windows Update hasn’t updated any new updates for past few weeks or if searching for new updates in Windows Update hangs forever or you run into some weird problems with installing Microsoft tools then you should read this article, as you may find fix for your problem and many useful links here.