How to delete Problem Report History in Windows 7:
Click the start buttotn --> the Control Panel (All Items View)--> the Action Center icon --> Click on Maintenance to expand that section --> the View system history link under the Check for solutions to problem reports section --> the View problem reports and responses link at the bottom of the Reliability Monitor --> At the bottom of the Problem History window, click on the Clear Problem History button --> Click on "Clear all" button.
Now all the windows 7 problem history is clear.
