Can I use my mid-2010 imac as a display for my PC with HDMI input?
Apple has just such a page in its knowledgebase on Target Display Mode.

But the one thing they do not mention is if it works with Windows (or other) PCs. It would make sense that it should/could work but I have no experience in doing so. Perhaps that article will help. perhaps others who have tried this will know for sure.