"Wireless HDMI" exists but it is expensive. That's the only wireless way to treat your HDTV as if it was a monitor.
Mediaboxes like the Boxee Box can use your PC as a hard drive and decode video and music and pass it on, but it won't simply show what is on your screen.
If you want to simply play videos and music the two best options are:
1) Western Digital TV LIVE HUB (internal 1TB hard drive)
2) Boxee Box (no internal hard drive)
The Boxee Box is excellent and probably would work well with a Western Digital 2TB Elements USB drive. I have the WDTV LIVE HUB and quite enjoy it.
Both boxes can use other programs like Netflix.
I have a small room with a computer, monitor, HDTV, WDTVLIVE HUB, Satellite box and BluRay player. Rather than send movies to my HDTV, I have them copied to my WDTVLIVE HUB which is connected to my HDTV. I can even copy videos to a USB stick and play it from my WDTVLIVEHUB.