If you are an English speaker, the easiest way to type Slovenian letters is to install a modified "+ Slovenian" keyboard layout.
There are modified US English, US international and UK English keyboard layouts available from
http://www.slovenian-translations.co...d-layouts.html
The layout is identical to your original layout, except that if you want to type Slovenian (and Croatian / Bosnian / Serbian) letters and the Euro sign, just hold right Alt + corresponding base letter:
AltGr + c = č
AltGr + s = š
AltGr + z = ž
AltGr + d = đ
AltGr + t = ć
AltGr + e = €
AltGr + 5 = €
Add Shift/CapsLock for upper case letters.
Additionally, you are advised to add Slovenian as an input language, so that Word's spell checker will know that you want to type in Slovenian. It's all described on the page above.
It works with Windows XP, Vista and Windows 7, both for 32-bit and 64-bit editions.
Happy typing :-)