Black screen after reboot

Dear users,
My system is configured as follows:
Dell Dimensions 8400, 8GB RAM, two hard drives with over 500 GB, one holds the operating system and the other data.
Windows 7 Home Premium
Nvidia GE Force 6800
About a day ago I rebooted and all I got was a complete black screen, no cursor no nothing.
I rebooted again and this time I was taken to Windows but at another resolution than my original settings and after going to display properties I noticed that my monitor was not recognized and the display was shown at default resolution.
Considering it may be a driver problem I updated the video card drivers to the latest available and also just in case, I changed my CMOS battery.
Once again I rebooted and this time I entered Windows normally, but after another reboot, the problem repeated itself again and most of the time I have to reboot two times in order to be successful.
To check if this was a software problem I restored my system from an image that was a month old and still the same situation.
At this point I don't know whether this is a software or hardware problem and I don't know what else to do for troubleshooting.
I would appreciate your advice.

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