You can use Windows 7 without activating for 30 days. However there's a way to extend the trial period by another 30 days. Actually this trick is supposed to work three times and so you can actually extend your trial to 120 days in total. As this trick would reset your trial period to another 30 days from the time it is performed, it is better to do this near the end of your trial period, this way you get the maximum possible time. So let's first check how many days you have left.
In the start menu right click on 'Computer' and click ‘Properties’ to bring up the properties window. Scroll down and at the bottom you should see the current product key and how many days are left to activate windows.
Now, here's how to extend the trial period. Click on ‘Start’, go to All Programs->Accessories. Right click on 'Command Prompt' and choose 'Run as administrator'.
In the command prompt enter the command 'slmgr -rearm' and hit enter. A message dialog should pop up saying the command has completed successfully. Restart your system and your windows 7 trial should be extended by another 30 days.