If you use WIN XP you can use whats known as Symlinking which creates Junction links that share info between different folders.

Im not THAT familiar with it but i have it set up where using a program called WINBOLIC I can have one complete install of WoW along with two very much smaller installs of WoW.

If you search on this site for the topic Winbolic guide you'll find and excellent expalntion with the link to its download.

So instaed of having 27 gigs of game you can now have 9.9 with three seperate installs running on one PC. Worked for me.