Transfering files across the network at slow speeds (and halts the internet)

[medley]

Strange problem really. I've had this on and off for a few months now. At one point it fixed itself and went back to normal but here's the situation. I have 2 computers on a wired network setup using Windows 7. I didn't do anything fancy in regards to setting it up as I just used the networking wizard (when I formatted my main computer recently). When its working fine I will get around 70/80mpbs transfer speeds across the network and will have no issues with regards to the internet. Alternatively, if its acting up like it has been in the past few weeks then I will get 11.1 mbps maximum speeds and my internet browsing will suddenly grind to a halt (take 20-30 seconds to load a simple website during the process)

I'm using the virgin media super hub and I'm on 50mb download speed. I'm not sure if its simply a setting I need to tweak in the router settings but if someone could shed some light on this I would be grateful