Ummm, you don't need to overwrite explorer.exe, you just need to change a per-user registry key. So no WFP.
Also, "replacing explorer" takes two parts: the shell ("desktop" if you want), and the directory/folder handler (the file-browser part of explorer). You can (and probably should
) use separate applications for this. Problem with the directory/folder handling is that things like My Network Places and other folders are special, and not all explorer replacements handle them well.
Didn't know that explorer (the shell) gobbles up around 20megs of memory, and was about to say "yeah, but you need to check private bytes, not working set size". But surprise surprise, at least on this Vista laptop, it's ~33megs of private bytes, ~22megs private working set size. Think I might be going back to blackbox on my XP64 workstation.