Reading from LAN PC slow, writing normal speed

[Remco32]

Out of the blue my freshly build HTPC can't be used to stream from. It is driving me nuts.
The HTPC is just a regular PC running W7 Ultimate with XBMC.
I can transfer any file to it and it will be transferred at the normal speed. If I try to open, on my main PC for example, any mediafile like an .mkv or .mp3 the item won't even load. If I open the HDD from any computer in the network browsing it is really slow: it takes forever to open a folder.
I think it must be the LAN, since the HTPC plays any file without a problem, and transferring files to the HTPC's HDD is normal as well.
HD Tune gives me this:

Any ideas? I am sure out of any.

 

[Remco32]

Forgot to add:

I also ran a Checkdisk which came out clean too.

 

[Remco32]

This happens when I transfer anything from the HTPC to my main PC:

Sorry for the triple post, I can 't seem to edit any of my own posts.

 

[Remco32]

I think I fixed it for now.
Follow the suggestions from this thread: http://www.sevenforums.com/network-sharing/2503-extremely-slow-file-transfers-network-access-4.html