Desktop hogging internet bandwith

Hi all,

I've been searching for a while and haven't found a solution. At home we have a TP-Link TD-W8968 as our router/modem.

Wired in is my desktop computer, and the rest of the family have WiFi connection too it. I get great speed on the desktop and so do they... when the desktop isn't on. The moment I boot up the wire connected computer everything else slows to a crawl or doesn't connect at all - phones, ipads laptops... the lot. The problem only appears to have eventuated in the last month or so. Antivirus hasn't turned up anything and I'm at a loss. Help?

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Professional, Service Pack 1, 64 bit
Processor: Intel(R) Xeon(R) CPU E5-1620 0 @ 3.60GHz, Intel64 Family 6 Model 45 Stepping 7
Processor Count: 8
RAM: 65462 Mb
Graphics Card: NVIDIA Quadro 4000, -2048 Mb
Hard Drives: C: Total - 242149 MB, Free - 78827 MB; D: Total - 1907599 MB, Free - 37472 MB; G: Total - 953866 MB, Free - 454386 MB; L: Total - 476937 MB, Free - 6391 MB;
Motherboard: Hewlett-Packard, 1589
Antivirus: Trend Micro OfficeScan Antivirus, Updated and Enabled