You're starting up your Windows (7, Vista, XP, NT, W2K, W2K3 or W2K8 PC or server) and everything looks normal. You log in, and your faced with a black screen with a single open My Computer Explorer window on it. You're wondering what happened.

Well, if this started happening after you updated your OS with the latest batch of security patches and then you tried to run a malware-removing software, the from Prevx will probably make it all better.

It seems that the problem has arisen because of a change to the OS' lock down of registry keys. Namely, the Access Control List, which is responsible of generating desktop features and interacts with said registry keys while doing it, stopped working as it should because this last round of Windows patches effected some changes to them.

Another option for fixing the problem is to manually modify the registry settings, but this should be done only by people who know what they're doing.

While investigating this issue, the Prevx team discovered there are at least 10 more different things that trigger the black screen. Their advice is to try their tool first because it can do no harm even if it doesn't do fix the problem.

UPDATE November 30, 2009: It looks like the problem is not related to Microsoft. More information at the MSRC.