Just remove the quotes inside your string, it should be the path, not a string of a string of the path. The spaces will be handled just fine.
"D:/Program Files/Steel Bank Common Lisp/1.1.12/sbcl.exe"
is how to specify that directory.
You should also look into adding the SBCL directory to your exec-path
variable. This is just like your PATH, Emacs looks there for executables. Add the directory to your exec-path
and specify your lisp program as "sbcl"
(add-to-list 'load-path "d:/slime-2.6")
(require 'slime-autoloads)
(add-to-list 'exec-path "D:/Program Files/Steel Bank Common Lisp/1.1.12/")
(setq inferior-lisp-program "sbcl")