Find your "Computer" icon, (used to be called "My Computer" in XP and earlier) and click it. That will show your hard drive usage and space.
Right click your hard drive and select Properties for more information.
I'm surprised you don't already know this. You have been using XP for years and Win 7 for some months now.
<font size="5">I have Windows 7 installed on my computer. What can I click that will tell me how many gigabytes of hard drive space that I have? Thank you. user470