I just happened to be working on my pc, deleting some old apps and things stored in My Documents.
I am an advanced pc user running Windows XP Pro SP2.
After deleting a lot of things I had stored in My Documents, I decided to reboot before running a defrag.
Upon rebooting, I got this black screen immediately saying "Error loading operating system".
I ran an advanced test on my Hitachi HDD using their Fitness Tool and no errors came up.
After thinking about it, I decided to boot up off my Windows OEM CD. At the menu I chose "R" to go into the Recovery Console.
At the prompt I typed "HELP", and it gave a list of commands.
I figured my Master Boot Record was corrupt so I typed "fixboot" and hit Enter.
I exited the Recovery Console, removed the cd and rebooted.
I would recommend trying this before trying most of the suggested options that come up when you Google this problem.
Edited by sungod, 17 May 2010 - 01:08 PM.