The 27" iMac shipped with a brand new feature: the ability to accept HD video from an outside source and display it on that beautiful 27" LED screen. A variety of devices can use this feature but perhaps one of the most frequently used sources is a PC laptop. While it might seem counter-intuitive that a PC can connect to an iMac as a display, it can easily be accomplished with the correct adaptor and a bit of technical knowledge.
Boot both the iMac and the PC Laptop. It will be handy if these two are sitting close to one another as you will need access to both machines for various steps.
Plug the Mini DisplayPort cable into the laptop.
Plug the other end of the Mini DisplayPort into the back of the iMac.
Set the iMac into "Target Display Mode" by pressing Command + F2 together. This will set the video source for your iMac to the rear port and prepare it for your laptop's video signal.
Configure your laptop to use the external display. Depending on your operating system this could be a number of different steps. Overall, you want your laptop's video signal to be ported to an external monitor. These options are often found in the video settings of the OS control settings.
Adjust the video settings from your laptop to optimise the resolutions of the iMac. These 27" iMacs have a maximum resolution of 2560 x 1440 pixels.
Don't worry if your laptop doesn't have a Mini DisplayPort. A quick web search will reveal many adaptor sources for VGA to Mini DisplayPort as well as HDMI to Mini DisplayPort.
Remember that the video in option only works for the 27" iMac.