strange letters when subs r bein used.

Hello!
I need help with one thing only.
When I watch a movie I use swedish subtitles so my brother can understand what is being said. A few months ago (before I re-installed my windows XP Profesional) I could watch the subtitles properly. But know the letter "Ä?" and "Ä?" can't be seen properly. Instead of these to letter I can only see some strange letter that doesn't even look like the right letters. I wonder why this has happened and how to fix. Can anybody please help me?

Best regards,
Romel