Just to make it plain:
There are channels that can stream content from one computer to another - this would be a file. It will have to be in the right format, and there are simple tools to do this.
Now, if you wanted to stream your entire desktop to see what is on your computer screen to the roku, this has to be done with live encoding, and the h.264 format takes some time to compress, so it is very sluggish, essentially slowing your computer down to where it isn't snappy anymore.
I'm sure we will get there, but not yet.
BTW, the only way around this that I can see is to put your computer video out into a hauppague hd external box and it will externally make an h.264 compliant file, and you would have to send that back in to your computer to stream that across the network using a roksbox/playon channel.