ViOrb

Get Vista's Start Orb on XP

ViOrb automaticly replaces your start button with one that looks the same as Vista's start orb. So you can get the Vista feel and look for Windows XP. Just download ViOrb, extract the ViOrb OneStep executable and then double click it.
Supports 3rd party skins

ViOrb by default comes with a Vista skin but it can support 3rd party skins available on deviantart. The screenshot shown on the right is part of fediafedia's Vista Live pack. It shows a ViStart and ViOrb custom skin by fediafedia.

Source : lee-soft.com