Hi
The Windows 7 Product Key (License) is perpetual, it never expires. You can reuse the key as many times as you wish, as long as the operating system is only installed on one computer at a time.
The Student version of Windows 7 is the Retail, Upgrade version. You will need to perform the installation the same way you did when you originally installed it.
The product key that you used to activate the first installation is kept on the Activation Servers at Microsoft. After you reinstall, you may need to perform a manual activation, which means that you may need to use the telephone option when you activate. Here are the instructions for a manual activation.
1. From the desktop, press the Windows Logo Key+R. This will bring up the Run dialog box.
2. In the Open: box, type slui 4 and press OK. This will start the manual activation wizard.
3. Select your Country in the drop down menu and click Next.
4. The wizard will display a Toll Free phone number. Leave the wizard open on the computer and call that phone number.
You will be given some recorded options. Select the option to talk to a live representative.
The representative will manually activate that system for you.
Let us know if you have any more problems