Could this be a reason why your computer is shutting off unexpectedly?
Log Name: System Source: Microsoft-Windows-Kernel-Power PM Task Category: None Level: Critical Keywords: (2) User: SYSTEM Computer: Owner-PC Description: The last sleep transition was unsuccessful. This error could be caused if the system stopped responding, failed, or lost power during the sleep transition. Event Xml:
You've tried a MSCONFIG option using selective startup and nothing changed. You could try to use System Restore Point to revert any changes made to your Vista computer. Also, checking your BIOS to make sure its not set to power off when in hibernation or sleep mode.
You can try entering your computers BIOS by hitting either an ESC, F1, or DELETE key when that computer first powers on. If your using a laptop, make sure you have your AC adapter plugged in or if that battery gets drained enough a default BIOS and operating system sleep settings will cause that computer to shut down. You could also try to update your video card drivers to their latest release from a manufacturer.
Making sure that you have Vista Service Pack 2 is recommended. One other tip is to make sure you didn't disable a Vista hibernation file when using Vistas system cleanup utility. Please check out this how to restore Windows XP to a previous state article for instructions on system restore.
Also, you can read how to obtain the latest Vista service pack article that will show you how to obtain Vista service pack 2. This Disk Cleanup Tool article explains how to make sure hibernation wasn't affected when running disk cleanup.