To enter diacritics in Windows I would suggest this keyboard layout. Alternatively, you can download Microsoft Keyboard Layout Editor from MSDN site and create a "layout of your dream" :), However it's a bit boring, so I'd stay with the first option (which essentially is the product of such a generator).
With this layout you could enter lowercase diacritics with <right-alt>+<base_letter> and uppercase with <right-alt>+<shift>+<base letter>. I use this layout when I write Spanish (huh, ¡Español! :)) and found it the most conventient solution for Windows, when you need to do a lot of programmings.
In Linux which is my primary desktop OS, I use the following xkb settings:
xkb_keymap {
xkb_keycodes { include "evdev+aliases(qwerty)" };
xkb_types { include "complete" };
xkb_compat { include "complete" };
xkb_symbols { include "pc+us(altgr-intl)+ru(ruu):2+inet(evdev)+altwin(left_meta_win)+group(lctrl_lshift_toggle)+level3(ralt_alt)+terminate(ctrl_alt_bksp)+keypad(pointerkeys)+level3(caps_switch)" };
xkb_geometry { include "pc(pc104)" };
};
This allows me to write English with all ' (quotes), " (double-quotes), and ` (backticks) as usual plus diacritics like á or ¡ by pressing <CapsLock>+<base_letter> (it's more convenient than <alt>+<base_letter>, because <alt>+<letter> are often used for shortcuts). Also I have a completely separate layout for Cyrillic (Russian + sometimes Ukrainian), turned on and off by left <ctrl>+<shift>