Major (or Fatal?) Error Message

The other week, after starting up my Dell Dimension 4500 desktop computer and popping off to make a coffee whilst it booted up, I came back to find an on screen message - something along the lines of: "Your Computer Has Recovered From a Major (or was it Fatal?) Error " and then I think it instructed me to click F10.

Since then it seemed to be OK but, when the computer has been shut down for at least 6 hours, when I boot it up again, it comes up with two warning(?) screens, as follows:

FIRST SCREEN:

From this screen, I click the DEL button and then it brings up a second screen:

SECOND SCREEN:

However, I'm not that smart when it comes to computer technicalities but I gambled on pressing the F10 button on my keyboard (as the screen shows: Save & Exit) and then my computer booted up properly and runs fine.

As I metioned earlier', as long as I boot it up again within a few hours after closing it down, I don't get the above screens. But if it's longer than a few hours -say overnight - then it does bring up the screens again.

Please, could anyone offer advice, help or whatever.

(p.s. I've tried System Restore and it's still the same problem)

Kind regards,

Tony