Can I reuse the serial number of Windows 7 Home Premium to reinstall my system?
The license key which found on the Windows CoA sticker on your machine usually at the bottom of the system, should be re-usable to activate the same version that previously was installed in your machine.
You may need to use the phone activation method to accomplish it, though.
"You can reinstall Windows 7 using Recovery in Control Panel, underAdvanced recovery methods. This method reinstalls Windows 7, either from a recovery image provided by your computer manufacturer, or from your original Windows 7installation files. You need to reinstall all of the programs that you added, and restore all of your files from a backup."