Peak Windows XP was Server 2003. I ran it as a daily driver on a ThinkPad. It could do pretty much everything XP could but had a closer UI to Windows 2000.

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djxfade 5 days ago | parent [-]

You could simply set Windows XP to use the classic theme as well.

	▲	EvanAnderson 5 days ago \| parent [-]
		Server 2003 had the NT 5.2 kernel. There were some minor improvements and it felt more stable to me than XP. The x64 version of Windows XP was based on the Server 2003 x64 build.