Re: Cannot load operating system! Please help!
Why do you assume there is one? Windows is not especially good at repairing the physical disk it's on if Windows itself can't load. Or that merely reformatting the drive will definitely work?
Sometimes things actually break. A few bad sectors in the wrong place can cause major problems that are fixable, but a head, local controller or cache failure means a trip to the parts store (and no way, shy of swapping out platters, to recover data from your disk -- external backups shouldn't be optional if you're doing anything important). And there's always the possibility that the mobo's SATA controller has decided to call it a day.
When you do remove the HDD, check it first before formatting. It may not be the problem, and there's no sense wiping everything just on a bet. Even with disk errors, the disk may be recoverable if there's a working OS with disk utilities in charge.
“Beware of bugs in the above code; I have only proved it correct, not tried it.” --Donald Knuth
"It was as if its architects were given a perfectly good hammer and gleefully replied, 'neat! With this hammer, we can build a tool that can pound in nails.'" -- Alex Papadimoulis (on TheDailyWTF.com)