If you are looking for a Windows Vista like experience on XP then you can try "Windows Vista Transformation Pack" shared earlier, but like me if you just want to get the cool looking functional Windows Vista sidebar and gadgets working on Windows XP computer, you should get the original Windows Vista sidebar tweaked to work on Windows XP by joshoon at DevianArt.
This original Windows Vista sidebar works fine on Windows XP provding full support for all gadgets from the Live Gallery, the sidebar makes use of Alky's DirectX 10 for Windows XP (included in the download) to make this functional on Windows XP.