Most system freezes (where you can't use hot-keys like CTRL+ALT+DEL) are caused by hanging drivers,
so you will have to either replace the device or update the driver. Troubleshooting can be done:
Download the setup from Windows Performance Analysis ToolsWindows Performance Analyzer for your Windows version.
Install the software on your system.
Open a command prompt as administrator, and copy paste the next command:
xperf -start perf!GeneralProfiles.InBuffer && timeout -1 && xperf -stop perf!GeneralProfiles.InBuffer myTrace.etl
Press ENTER once to start the command, now you will have to wait till your system hangs.
You can do whatever you want to. Please no heavy activity like gaming or private things...Right after your system stops hanging you go to the console and press ENTER.
After waiting some time a log file myTrace.etl will be produced, compress this to a zip file.
Put this compressed version of the file somewhere online (perhaps 2shared).
Share the link here, I will do an attempt to find and show you the cause of your problem.
OR troubleshoot the trace yourself using instructions found here, start with step 6 - 7.