If the computer is not used for a while (2-3 hrs) a band comes up showing a program has caused Windows to stop working. I have had this quite a few times. What causes it? Sent the details to Microsoft as requested every time but it keeps happening.

I have a few questions first, to get a clear picture of the issue:

1) What is the exact error message that you get? Do you get an error code?
2) Does this happen when you are using a specific program?
3) What happens when you get this error? Do you face any issues?
4) Do you remember making any changes to the computer prior to this issue?

This issue could be caused if any background services are conflicting with a program. I would suggest you to try these steps and check if it helps:

Method 1: System maintenance troubleshooter

The built-in System maintenance troubleshooter can detect and fix any incorrect settings or any errors with the Internet Explorer. You can refer to these steps to run the troubleshooter:

a) Press Windows key +X, select Control panel.
b) Change the view by option on the top right to Large icons.
c) Click on troubleshooting and click on the view all option on the left panel.
d) Run the “System maintenance” troubleshooter.

Method 2:
Step 1: Safe mode
I would suggest you to boot the computer in safe mode and check if the issue persists. Refer this link to boot the computer in safe mode:

http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode

Step 2: Clean boot
If you don’t face this issue in safe mode, then we will have to perform a clean boot procedure to find out which service is causing this issue. Refer this link to perform a clean boot:

http://support.microsoft.com/kb/929135

Note: After troubleshooting, reset the computer to start as usual. Refer step “How to set the computer to start as usual after clean boot troubleshooting” under more information.