Today I've been trying out XFCE 4.8 on Debian Wheezy from a Live USB. It zips along even from the USB, so I have no doubt it would be fast if installed- just about ideal for this or any other XP era machine.
The only real drawback with XFCE is that it is a 20th century desktop paradigm. Some see limitations in that paradigm, others comfort. The limitations are a potential dog's breakfast in the notification area, redundant icons with duplicated functionality, inefficient use of screen space, and inefficiency in a cluttered window switching mechanism.
It seems XFCE has tried to get away from the Windows 95 paradigm with the main panel now positioned at the top of the screen, and a second panel acting as an application launcher at the bottom of the screen.
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