If removing the network cable suddenly speeds things up, then, you're the victim of an unknown Windows bug. For example, when you right-click in Windows Explorer, does it sometimes hang?
Also, is this a computer that's run on more than one network? This happens especially on laptops that have visitied multiple lans (including your home)
You PC's Registry, or something, has Vestiges of Pointers to network resources (not just shared drives) but Icons, things it thinks are out on the network but are not there anymore. You have to wait for the time-out to finish before you have response.
I believe this is also causing slowness in general when these timeouts are in series. Just an boatload of waiting for network things it will never find.
I completely wrote my own Window Explorer where every resource request is done in it's own thread. Any thread that doesn't complete within 1 second is ferociuosly killed. End result, I have my own custom Explorer and no delays. Doesn't have all the bells and whistles, so I haven't shared it.
This really sucks... Best of all! None of the Registry Cleaner Tools, spyware tools will solve this.