Click on Start

All Programs


Command Prompt

Type or paste the following command : slmgr -dli

A pop up box will appear, and it will have the Time Remaining period as shown below

To extend ( reset/rearm) the evaluation period to another 30 days,

click on Start 

All Programs 


right click on Command Prompt

and click on Run as administrator

If you have the User Access Control feature set as default, then a pop up box will appear and you will have to click on Yes to confirm the action

Type or paste the following command:

slmgr -rearm

A confirmation message will be displayed, stating that the command completed successfully, and that you will need to restart the machine in order for the changes to take effect. So reboot your machine.