Can I use an iMac as an external monitor if not booting up the OS? My 2012 iMac was in a room with room temperature of about 39 C, and now the hard drive seems to be not working well, taking 5 minutes to launch an app.  If I boot up Win 7 using bootcamp, Win 7 reports that the hard drive might have problems.

I wonder how high the cost of repairing it, and I might get a new iMac, so a plan is, to use the old iMac as an external display for my Macbook Air.

But is it true that I must turn on the iMac and boot up Mac OS X or Win 7 in order for it to act as an external monitor?  It will just be boot up the OS and I won't use it since it is not functioning, so I would like to find a way to somehow just use it as an external display but without having the processor and RAM all be consuming power.

(a partial solution might be to turn on the iMac while pressing the Option key, so that it will let me choose the OS to boot up to, and the display might be able to be used, without any OS running.  But the processor and the RAM probably will still be consuming power).
Based on the Apple Support Doc http://support.apple.com/kb/HT3924 it doesn't look like it. But if it is the hard disk, it might be cheaper to replace the hard disk, then to buy a monitor.