Security experts from McAfee found an embarrassing bug in Windows Vista. They say it's possible to crash the operating system and put it into an interminable crash-restart-crash-loop by trying to load a malformed animated cursor file:
It isn't even a new exploit, as researchers with eEye discovered in January 2005. At that time, Microsoft acknowledged it affected versions of the operating system from the first edition of Windows 98 through to early releases of Windows XP, though it stated at the time XP SP1 was unaffected.
But apparently after researching field reports of limited attacks, Avert Labs discovered an apparently similar exploit using .ANI files impacts XP SP2 and Vista as well, as well as Windows 2000 SP4 and versions of Windows Server 2003 from the initial release through to SP1. Avert Labs stated XP SP1 and versions since were unaffected, though Microsoft warned the exploit does affect XP SP2.
If both firms' accounts are correct, Microsoft may have fixed the problem with XP SP1 in 2005, and inadvertently un-fixed it sometime afterward.
Avert Labs' video of the incident, posted to YouTube, shows a Vista system wherein the test file apparently trying to load the custom animated cursor. When the operating system detects a crash, it first tries to save vital data prior to a restart sequence - one of Vista's newer features. It then informs the user that Windows Explorer has crashed.