See the 3rd reply in this old forum thread by "bugsbunny"
archives/PT2/viewtopic.php?t=17740&highlight=move+postgresqlwhich details how to do this.
[bubo]MAKE SURE YOU MAKE BACKUPS OF YOUR DATABASES FIRST!!!![/bubo] I can't stress this enough. To be extra safe, I would also suggest you ZIP the contents of the c:\program files\postgresql\8.2\data folder first as well just to have an extra backup because if doing what it says in that post above does not work for you then it would probably just be easier to reinstall PostgreSQL on the drive you want it to be on and then restore from your backups.