Use MacBook Air as a monitor for Win7 Desktop PC
According to Apple's documentation notebooks do not support Target Display mode.

However, if you run any kind of VNC software on your Windows machine you can use the Windows desktop from your Mac using the Screen Sharing application. No additional software is needed on the Mac end.

If your Windows machine won't boot to the desktop the VNC software will not run, so you will need a temporary monitor here. And you will need to install the VNC program somehow. 

In short, Windows computers really expect to have a keyboard, monitor and mouse connected.