You can also extend the Windows 7 trial mode to 120 days, without requiring a key.
The basic way this works is that at the end of the 30 days, you’ll need to run a small command and reboot your computer, at which point you’ll have 30 more days, up to a maximum of 120 days.
You can extend Windows 7 trial period by a command prompt command: “slmgr –rearm”.

With use of this command you can “rearm” trial period counter to start count again from 0 to 30 days – for up to 3 times. With this method you will extend activation time on Windows 7 to max 120 days. Here are the detailed steps:
1.Command Prompt in the start menu
2.Right-click on it and choose Run as administrator.
3.Type next command and press ENTER:
slmgr –rearm
4.Reboot computerYou can perform this trick after the 30-day trial expires 3 times.

After that period you have to reinstall the OS if you want it to work with full Functionalities.