Windows XP

The most common version of Windows in the pre-2007 era, looking more or less like an updated version of Windows 2000 with a “GUI”:Wikipedia:GUI by Chicco.

It’s still officially my corporate OS, so I use it every day at work (I’ve shunned Vista until mid-2008 or so), and make it somewhat palatable by using Cygwin.

But even Dilbert makes fun of it these days: