Are you sure you are not pressing Shift and then 5? You MUST press Ctrl then Alt, and while holding them down, press the 5 button.
Alternatively, if that still doesn't work, you can do it another way. First, make sure Num Lock is switched on. Then, hold down the Alt key, and keeping it held down, press the numbers 0 1 2 8 in order on the right number pad. Let go of the Alt key. This should type a Euro symbol for you.
-tigermatt