//Escape backslashes with an extra backslash (\\)
if(clr.System.IO.Directory.Exists("C:\\Windows")) {
    sp.MessageBox("C:\\Windows Folder Exists", "Found");
}

//Note the ! before clr, means NOT
if(!clr.System.IO.Directory.Exists("C:\\DoesNotExist")) {
    sp.MessageBox("C:\\DoesNotExist Folder Does Not Exist", "Not Found");
}

//Can compare to true, but means the same
if(clr.System.IO.File.Exists("C:\\Windows\\win.ini") == true) {
    sp.MessageBox("C:\\Windows\\win.ini File Exists", "Found");
}

//Instead of ! can compare to false
if(clr.System.IO.File.Exists("C:\\Windows\\DoesNotExist.ini") == false) {
    sp.MessageBox("C:\\Windows\\DoesNotExist.ini File Does Not Exist", "Not Found");
}