1) Backup any important files on your OS HDD, in case it is actually beginning to fall.
2) Check the Windows System Event Log for Disk Errors.
Do you have Western Digital or Seagate HDD's? Run an HDD diagnostic utility such as Data Lifeguard for WD or SeaTools for Seagate (the extended test on a bookable CD) to determine if your HDD's are sound. If your OS HDD fails the test, replace it.
3) If it passes the HDD diagnostic test, run CHKDSK C: /F from an elevated Command Prompt (right-click on the Command Prompt icon and choose "Run as administrator" from the popup menu) to confirm your file system is sound.
If file system errors are found, run the Windows System File Checker command, SFC /scannow, also from an elevated Command Prompt, to eliminate possible corruption in the Windows System files.
4) Load up HWINFO64 and configure it to log CPU and GPU temperatures to determine if you have an overheating issue. Note your laptop's idle temperatures (no other applications running). If your laptop shuts down unexpectedly, restart it and open the log file to determine the maximum temperature recorded by your processors.
5) As recommended by @fostert, run memtest86+ for at least 3 passes to determine if your memory is sound.
G74SX-A1 - stock hardware - BIOS 202 - 2nd Monitor VISIO VF551XVT