I do not think this is the answer you were looking for.
Unfortunately it is not possible to use an iMac as a PC Monitor, even if the PC supports Thunderbolt. The iMac can only be used in this way by another macintosh computer such as a Macbook Air or Macbook Pro (Target Display Mode). Very few PCs support Thunderbolt anyway and I don't think anyone has found a way to force this to work.
The lack of this functionality seems to be a bit of an oversight by Apple, however you have to remember that when buying Apple Computers they want you to buy their equipment and do not see the need for you to own a PC or something which may want to use the display other than the iMac itself or an Apple Laptop.
This is to the best of my knowledge, so someone please correct me if I'm wrong.
The really annoying this is that you can use the thunderbolt display as a PC monitor, so why not the iMac? People need to complain directly to Apple, as this could be an oversight that's fixable with a firmware update.
The really annoying this is that you can use the thunderbolt display as a PC monitor, so why not the iMac? People need to complain directly to Apple, as this could be an oversight that's fixable with a firmware update.