It may be because your text editor does not recognize properly the character encoding, and automatically opens the file with latin CP1252
So you need to open the file with a text editor that lets you chose the encoding, eg Microsoft Word.
Open the sub file with MS Word, and select the encoding that will show correctly the text.
Then select UTF-8 encoding when saving the file to avoid further problems. UTF-8 encoding has the advantage over 8-bit encodings, to be automatically recognized by the vast majority of text editors.
Note: if you video reader does not read UTF-8, then it is time to change to a recent one...
EDIT: If you have weird characters using VLC player and don't want to manually convert your subs each time, go to Tools->Preferences->Subtitles and change the default encoding to your preferred one (eg Windows-1250 for Eastern European languages).