Connecting iMac + External Monitor to Win 7 Desktop
Target Display Mode is the term.  Seems every iMac has a different way of handling this.  

If the Win7 machine has a display port output, you should be able to do this?

Here's some more info

Can I use the iMac in TDM as a display for my gaming console, DVD player, or other device?
The Mini DisplayPort port on a supported iMac can receive DisplayPort compliant video and audio signals. Converters not made by Apple may provide options to convert other electrical, video, and audio protocols to Mini DisplayPort or Thunderbolt compliant signals.