Some of us spend a lot of time trying to elevate our status in life and accumulate more privileges. Apparently, that's not as as difficult as it should be (it should be impossible) in the Windows XP and Server 2003 kernels. With the proper knowledge, you can log on as a restricted user and exploit a vulnerability to gain admin rights and control over everything on the system.
Microsoft is investigating, but don’t expect this kind of service for much longer. The company will be phasing out support for these operating systems early next year. If you're still on Windows XP (or know someone else who is), it's time to upgrade. Seriously.