One of my Dell Optiplex 380 systems has been working fine for over 3 years now.
Today I turned it on and it had a black screen with a flashing cursor.
After a few minutes, I received the message "Error loading Operating System".
Using a different hard drive from an exact same system (I have 5) - same message.
I tried to run a repair from a Windows XP Pro install CD and I got the same message.
The Dell diagnostics (pre-boot) showed no problems with the hard drive or memory.