Reports are coming in from users who found themselves unable to boot into Windows after installing this month's security patches. The affected machines include some Windows 7 and Vista clients and Windows Server 2008 servers. This doesn't happen to all computers running those operating systems; it seems to be related to Kaspersky anti-virus software and a specific update that fixes an "important" (but not critical) issue. If you get hit, you'll get a STOP error (blue screen of death) and the computer will reboot, only to encounter the same thing.
The good news is that you can uninstall the update if you haven't rebooted the computer yet, and avoid the problem. If your machine is already in the loop, you may have to do a system restore or remove the update manually from the command line.
Read more here: