You could try a browser plugin that will hide you windows if you press a button or use a keyboard shortcut:
A different approach is to browse the web as normal and then hide all the evidence as soon as anyone wanders over to your cubicle. Win+M minimizes all open windows on a Windows machine while Cmd+H hides the current app on OS X; you can of course also quit the current app with a keystroke (Alt+F4 on Windows or Cmd+Q on OS X).
Boss Key lets you set your own shortcut and hides everything from the Windows taskbar as well, making it impossible for anyone looking over your shoulder to see what you’ve been up to.
Don’t Panic! for Windows does a similar job but cleans up your recent browsing history at the same time.
I've not tried any of these myself as I work from home. Although I do a lot of screen sharing sessions and I've gotten a few poorly timed skype messages before.