Media Server? How?

I just purchased a Roku for streaming content of the internet, netflix, amazon, etc. I also bought a new wireless router. I have the wireless network set up just fine in my house. The Roku can go online as can everything else. I have attached an external HDD that I have loaded with movies to the router. My laptop and pc can see and access this drive just fine.

According to Roku I can stream content from this drive (one reason I bought it).

Here is my question/issue. In order to stream I need to set up a media server. How do I do this? I want to set something up on just that drive so that I can leave my pc off. The few things I have found online seem to assume that I am leaving my pc on and streaming off of that.

With a roku you can't you need to buy and apple tv that's the only way you could accomplish that task

With a roku you can't you need to buy and apple tv that's the only way you could accomplish that task.

With a roku you can't you need to buy and apple tv that's the only way you could accomplish that task..

With a roku you can't you need to buy and apple tv that's the only way you could accomplish that task...

With a roku you can't you need to buy and apple tv that's the only way you could accomplish that task....