pimp my desktop: how to?

i am getting tired of my standard windows looking pc, and apart from changeing the backround i have no clue about changeing themes

I here windows xp theme from microsoft is pants and uses up to much memory?

i want something abit bling like aston or w/e

but i have no clue on how to do it or what i need or what it involves (will i have to reinstall any programs or changing directorys?)

i am currently running windows xp profesional service pack 2


so if any one can help let me know, if your feeling up to it you can also pm me in #xytogen thanks