Luna Element is probably the best visual style for Windows XP out there. I’ve been using it since version 3. Luna Element 5 was released today.
It’s available in the same blue and black variants as Luna Element 4. There are a few subtle changes since Luna Element 4 and some more noticable ones; highlighted items and text now have a light blue background, the start button is circular and the scrollbar changes colour when you move your mouse over it.
At the moment I would still say version 4 was better but I need a little bit of time to get used to this new version. I remember saying the same about version 4 when it came out but it grew on me. I expect this will also grow on me.
Highly recommended.
I think it’s worth mentioning that you need to patch XP’s uxtheme.dll before you can install third party visual styles.