Depends what it says on the screen, I've had it happen and it was the hard drive but I've had it on a work computer when it was the graphics card died, so depends what the error says.
The most important Blue Screen of Death troubleshooting step you can take is to ask yourself what you just did.
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.
Undo the change you made and test again for the STOP Error. Depending on what change you made, some solutions might include:
Verify that a minimum amount of free space is available on your Windows partition. Blue Screens of Death and other serious issues, like data corruption, can occur if there's not enough free space on your primary partition used for the Windows operating system. Note: Microsoft recommends that you maintain at least 100MB of free space but I regularly see problems with free space that low. I usually advise Windows users to keep at least 15% of a drive's capacity free at all times.
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. Important: Make sure your virus scanning software is completely up to date and that it's configured to scan the MBR and boot sector.
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.
Return BIOS settings to their default levels. An overclocked or misconfigured BIOS can cause all sorts of random issues, including BSODs. Note: If you've made several customizations to your BIOS settings and don't wish to load the default ones then at least try returning clock speed, voltage settings, and BIOS memory options to their default settings and see if that fixes the STOP error.
Make sure all internal cables, cards, and other components are installed and seated properly. Hardware that's not firmly in place can cause a Blue Screen of Death so try reseating the following and then test for the STOP message again:
Perform diagnostic tests on all hardware you're able to test. It's highly likely that the root cause of any given Blue Screen of Death is a failing piece of hardware:
Update your BIOS. In some situations, and outdated BIOS could cause a Blue Screen of Death due to certain incompatibilities.
Start your PC with essential hardware only. A useful troubleshooting step in many situations, including BSOD issues, is to start your computer with the minimum hardware necessary to run the operating system. If your computer starts successfully it proves that one of the removed hardware devices was the cause of the STOP message. Tip: Typically, the only necessary hardware for starting your PC through to the operating system includes the motherboard, CPU, RAM, primary hard drive, keyboard, video card, and monitor.
Is your PC restarting before you can read the STOP Code on the Blue Screen of Death?