If it has Win 7 Pro, then it has Remote Desktop Server. You'll need to temporarily install a video card (can borrow one from another computer) and hook it up to a monitor. Once you've booted up, go to:
Start -> right-click Computer -> Properties -> Remote Settings
In the bottom half of the dialog box, you'll see options for Remote Desktop. Enable it. If you're running Windows 7/8 on your main computer, pick the more secure option with Network level authentication. Now make sure to write down the computer name, it's IP address, the account's username, and give it a password if it doesn't have one already. Shut down the computer.
You can now put the video card back in your other computer. Start up both computers. Wait a minute or two for the monitor-less desktop to finish booting. Once it's ready, start up Remote Desktop Client on your main PC (all versions of Windows come with the client). If you can't find it, just searching for Remote Desktop will usually bring it up.
Run it. When it asks which computer to connect to, enter the monitor-less computer's name. You can change some of the other options too while you're at it. I like to run it in a window instead of full screen. When you connect, you should get a box asking for a username and password. If it doesn't connect, try using the computer's IP address instead of name. Enter the username and password. You will now see the monitor-less computer's desktop on your main computer.
You can proceed to use it just as if you were using it directly. The only real restriction is that you can't shutdown directly over remote desktop. But you can get by this by Start -> Run -> type 'shutdown -s'