1) Did you just install a new program or a piece of hardware, update a driver, install an update, etc.? If so, there's a very good chance that the change you made caused the BSOD:
Startup using Last Known Good Configuration to undo recent registry and driver changes.
Use System Restore to undo recent system changes.
Roll Back device driver to version prior to your driver update.
2) Scan your computer for viruses. Some viruses can cause a Blue Screen of Death, especially ones that infect the master boot record (MBR) or boot sector.
3) Update drivers for your hardware. Most Blue Screens of Death are hardware or driver related so updated drivers could fix the cause of the STOP error.
4) Return hardware settings to default in Device Manager. Unless you have a specific reason to do so, the system resources that an individual piece of hardware is configured to use in Device Manager should be set to default. Non-default hardware settings have been known to cause a Blue Screen of Death.
5) Return BIOS settings to their default levels. An overclocked or misconfigured BIOS can cause all sorts of random issues, including BSODs.