Is there any way to use a 2011 iMac as a monitor for a non-Thunderbolt PC?
Sorry, no. There are some PCs and motherboards coming out now that support Thunderbolt, but because of how Thunderbolt works, it's not something that can be added on later to something that didn't initially support it.
Although it is an entirely different direction, you could run a VNC or RDP server on the other machine, and run the client on the iMac. It's not like plugging in a monitor per se, but it does allow you to have another computer show up on your iMac's display.