On some servers you need the / , on others you don’t, depending on the version of ISAPI_Rewrite.
\/ is (normally, not sure about ISAPI_Rewrite) just another way to write / (where \ means: take the next character literally)
By the way, on some servers it takes a while before changes to that file take effect. You may want to contact your hoster.
Apache 1.x required a leading / in the DocumentRoot’s .htaccess mod_rewrite directives; Apache 2.x KNOWS that the / is in the URL (not the URI) so it’s effectively prohibited. The result is: