Tremendously slow transfer rate on one PC but not others.

Let me explain my situation, as it's rather complicated.

PC A: My primary PC. It has 2x HDD (500 gb and 1 tb) and 1 120gb SSD. My OS is on the SSD. My data is on the 500 (including media), and my programs are on my 1 tb. Connected LAN to the router.

PC B: My wife's PC. Connected LAN to the router. Windows 7 Ultimate X64

PC C: Living room PC. Connected Wireless N 300 to the router. Windows 7 Ultimate X64

PC D: Laptop PC. Connected wireless G. Windows Vista Home Basic

The issue: I cannot play my videos stored on PC A on PC C. Nor can I play them on PC B. I can, however, play them fine on PC D. I can also play videos from PC B on PC A fine. In fact, the whole network access is slow when viewing PC A from B or C. When I click the video, it lags heavily, wont play the video, and locks up windows explorer.

Steps I have taken: Re-installed windows on PC A. Turned off Windows Firewall, disabled autotuning, removed RDC. Replaced modem (old: DGL-4300, new: ASUS RT-N66U).

None of these PCs have obsolete components, and it's strange that one configuration doesn't work properly but the other does. Please offer some suggestions?

Edit: PC C can access PC B just fine. So here's the break down:
PC A: Accesses all fine.
PC B: Accesses all but A fine
PC C: Accesses all but A Fine
PC D: Accesses all fine.

Edit: It also does this for audio and image files.

How would I go about doing that?

Just a hint, it's slow to access any drive. Not just my media drive. I've tried putting the files on another drive and accessing them. Same result. Plus, the laptop accesses them just fine.

Oh my god, I just fixed it. I updated my network adapter drivers in device manager. I'm going to go beat the hell out of myself for about 3 hours for spending the last 7 hours working on this. Thanks for trying to help anyway.

It did. Works like a charm now.