Ok, this one made me say wtf out loud. I am possibly including more detail than the story requires because I would be interested if someone could explain how this happened, or possibly tell me how I'm TRWTF.
I was asked by a friend to fix their computer. The graphics card (Nvidia Geforce 5500 PCIe, old I know but still) had stopped sending a signal to the monitor. The monitor simply reported 'Signal out of range' whether the computer was switched off or not but the system was working - you could hear it start up Windows XP and shut down following the correct key presses. I removed the graphics card, gave it and it's connectors a thorough clean,and reconnected it. No change. The motherboard had an internal graphics adaptor (some kind of Intel Integrated Graphics, I didn't pay much attention to exactly what) so I again removed the Nvidia card and reset the BIOS so that it would use the internal card (had to remove cmos chip because there was no screen otherwise). this worked fine, BIOScame up, booted into Windows with no problems, if a little low resolution. All good.
Next thing I did was ensure he had the drivers for his card (a file on the harddrive, latest Nvidia drivers) and completely remove the installed drivers. I then rebooted into BIOS and swapped the video configuration over to PCIe, switched off, reinstalled the card and rebooted. All fine, video works fine, Windows boots up and finds new hardware. I cancel it's wizard and run the setup program from the hard drive. Now, this is where it gets a bit iffy. When I attempt to run the setup AVG reports it is virused. I remember hearing about this, and also that the general consensus was that it is a false-positive, and since he doesn't actually have internet access at the moment I go ahead anyway. Installs fine, asks me to reboot, reboots fine except that it is now asking me to run chkdsk. I know I should let this run, but I figure he can do it later, I don't want to be here all day so I cancel it.
Now, here is where I say WTF!. His monitor screen stays on the blue chkdsk screen, but the computer boots into Windows. I hear the start up sounds, I make the correct key presses and I hear the shutdown sound, computer shuts down and the screen returns to Signal out of range. Note: right up till the monitor reports signal out of range it is still displaying the blue chkdsk screen with 1 second left to cancel before it starts.
I start the computer again and this time allow it to run chkdsk. This runs fine, it reports some lost data from the AVG installation and recovers a few sectors, then displays the XP loading screen. Then the screen goes black. No picture, no signal out of range message, but it still boots into Windows. Again I close it down, and now the card has completely stopped working again, signal out of range message even though Windows still boots up.
I repeat the whole process from the start, still using the same potentially virused driver, and this time everything works perfectly. I get the system competely up and running and it seems ok. To finish, I advise him that he needs to completely clean his system preferably with a reformat and complete reinstall and I will download the graphics drivers anew for him.
So, can anyone explain how the blue chkdsk screen was able to remain visible even though the system had booted into Windows?