1) Download a visual style based on Vista, such as
VistaXP.
2) Get some icons to replace the current XP ones, from a site such as
Vista Icons. (Not sure how to inject these into the DLL files though...anyone? )
3) Get some Vista wallpapers. Pretty much just Google something like "Vista Wallpapers" and you'll come up with them. Or you could get your own closeup high res images of grass or plants.
4) Play around. You can find themes for Firefox, Winamp, Windows Media Player, etc. Try a site such as
devaintART for more.
There are programs available, such as the
Vista Transformation Pack, however unless you don't mind breaking your XP install, or are planning to leave it like that until you next reinstall XP, I'd stay well away from stuff like that.