hard disk is the laptop's main memory device, the place where everything is stored into. Even files needed to run Windows 7 at all.
When you turn on you should see some white text, and among that there should be a line telling what button to press to get into BIOS settings, it is usually F2 or F12 or Del, be very quick and press it multiple times just after you turn it on.
Once you are in the BIOS settings screen navigate around it with arrows and enter button, find the page that talks about hard drives and see if it lists a hard drive in there (the name should look like a bunch of letters and numbers and maybe some hint at capacity and manufacturer, like this: HTS727550A9E364)
Then do what Pauly says.