Character display errors on email and websites.

I have a unique problem related to character display
errors only when I'm using the Internet. I first noticed
it when I sent email to friends using Yahoo, but not
HotMail. My apostrophe key (both symbols) and the dash
key (both symbols) will be viewed as Greek letters to
the recipient of my email - but not on my screen. If I
send myself an email to another one of my Yahoo email
addresses, my message looks OK on my screen - but not
anyone else's. I never seen the greek characters on my
screen.

I also noticed on some websites, I view the umlaut sign
(double dots above the font character) above the vowel
letters, like an (A or U). When I view the same website
on another computer using Windows XP, I don't see the
umlaut signs.

Please help me - it is driving me nuts.