your got to reformat your computer.. it means your hardrive has crashed and the only way to fix it is to reformat.
Not always true, did you changed your hardware or anything? What error code does it give you on the bluescreen.
Like everyone says here it can be caused by severals things.
The issue I often see is someone trying to start windows on their workstation, but it doesnt support SATA HDD. Which will cause a bluescreen. (Their windows version doesnt have the driver on the cd i.g, won't go in detail.)
To fix it I go in the bios and emulate the SATA with IDE.
This is one case out of many. But depending what you have done maybe you can try this.