The basic answer is no... the longer answer is that you might be able to do it using something like an EyeTV (like a TV card that accepts a digital signal), but it would be a pain.
Better options...
1) Run the PC as a server (headless) somewhere on your network. Use a remote desktop client to access it from the Mac if you really need to.
2) Run Vista and OS X both on the Mac, using either Bootcamp or Parallels (I understand that, at the moment, there might be some issues with the 24" iMac's display and Windows? But in general, many people are happy with this solution).