Transfer rate drops when not using the pc

[pMyzJC]

Long time user, first time poster.

If I leave my laptop going with uTorrent up I find I get far slower transfer rates than if I'm at the computer doing something else at the same time. It's sometimes as much as half the speed. I suppose it's something turning itself off or slowing down to preserve power but I'm buggered if I know what it is. I didn't notice the problem with my old laptop but it seems to happen quite consistently with this newer one.

The speed in general is pretty good and I'm happy even with the slower speed but can't help but feel that this is going to be something simple. When using the computer I sometimes get speeds over 700kB/s but when I leave it for a while and go off and do anything else it is rare that it will touch 400.

Haven't done port forwarding as the speeds are generally better than they were with my old laptop on which I had forwarded the port (is this the correct parlance? not important).

Details the forum requests:

The network status light is green throughout

speed test says u/l 524.28kb/s, d/l 556.8kb/s, current settings 376.83kb/s

port checker says port is open, automatic port mapping on

net.max_halfopen=100

no idea wtf TCPIP.sys is so I'd imagine I've never modified it

OS= Windows 7

Bullguard firewall installed, windows firewall also on (is having both on pointless?)

Router= Thomson speedtouch 585 v6, connected via wi-fi

Modem= dunno

ISP= cable & wireless

Connection type= dunno, DSL? That's a guess.

Any ideas please?