Hi Fraser,
Certainly seems as though the Zyxel is giving priority to your desktop. Maybe the control protocol says to itself "Desktop... MASTER SPEAKING... must obey" (*only in binary*) OK that's silly, I know, but I've just googled the 334WT Zyxel and found this interesting paragraph:
"The router also has bandwidth management built into it, thus allowing you to set priorities for different applications like Voice over IP and streaming media. To make life easier on the wireless side, as well as supporting the usual WEP and WPA encryption standards, the router has OTIST (One-Touch Intelligent Security Technology) which makes it easier to create a secure network between different ZyXEL wireless products that support OTIST. The usual Universal Plug and Play (UPNP) is included too. "
Have you maybe been using VOIP, Skype or one of its friends? If the Zyxel is plug-and-plop, as it seems to be, you may need to get into the setup pages and force it to behave itself. (192.168.1.1 or similar should get you there)
Will follow the thread with interest: this could run and run - tho' I hope not
Jim