I've been using shell replacements for a really long time now ... been using them from old Win98 days...
There are lots of shells available these days ... to name a few Litestep,Darkstep,Aston shell,DesktopX,Talisman etc..
Personally i prefer Litestep..... It has a very small footprint,will run on most systems,very very customisable n' has tons of modules available for it.
You can have a look at my curent desktop which i've built on Litestep itself , it's based on a theme from Talisman. If you have any more queries on this post em'!