I'm using the 'Royale' theme for xp - apparently it's one ms were going to include but didn't in the end, or something, but you can download it and install it and it doesn't have any slow down at all, because it's just like switching between the 'olive' and the 'blue' theme in the display settings (It comes up as another option). It's not amazingly differnt to the standard setup, it's more of a subtle change, but if you've only ever used the standard setup than you might like it.
There's two actually, 'Royale' and 'Royale Noir' - don't like the noir one much, they didn't get it finished so some things don't look quite right (like the edge of the start button). The 'Royale' one looks fine though.
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"If at first you don't succeed, remove all evidence you ever tried"