Easy regexp issue

[HS]

Hello everyone!

I'm finding this quite odd:
I want to replace all slashes for backslashes in a string, but i'm
having trouble!

Running this:
(replace-regexp-in-string "/" "\\" "c:/emacs/lisp/")
I get:
Debugger entered--Lisp error: (error "Invalid use of `\\' in
replacement text")

And if I try:
(replace-regexp-in-string "/" "\" "c:/emacs/lisp/")
I get:
Debugger entered--Lisp error: (scan-error "Unbalanced parentheses" 302
1)

So, I don't see any other options!
I know the problems is there because I replace for another string then
it works:
(replace-regexp-in-string "/" "x" "c:/emacs/lisp/")
"c:xemacsxlispx"

So, any ideas?

Thanks again!
HS