Upload bandwidth low compared to other OS

I've been prepared to blame my ISP for my poor upload speeds, but today I realised it is only Windows 8 Pro with Media Center (my main OS) that is having the problems. Linux Mint, and an old Windows 8 CP Windows To Go USB stick both achieve the upload and download speeds expected on the same laptop. I've tried a range of browsers, and they are all similar in the same OS environment. I have tested with several different speed testers, but I'll illustrate this with ThinkBroadBand's speed test.

What I should get is up to 80Mbps download and uploads up to 20Mbps.

Under Windows 8 Pro with Media Center I get this:
very low upload speeds about 1% expected, good download rate dropping off when the connection is saturated

Linux mint:
consistently good rates especially under high throughput connections

Windows 8 CP To Go:
generally high rates, but uploads drop off under higher throuput

Does anyone know how to change the characteristics to improve my Windows 8 performance?