my emails come back with symbols where the apostrophes should be

[scelek]

I keep getting my emails back with fonts I didn't use, three symbols (’)
where apostrophes once were, and my signature comes back in reply emails
formatted differently and partially in different fonts. What can I do to
stop this from happening? I understand that this is a character encoding
problem, but I don't know what to do about it - I recently switched over to a
new computer and never had this problem on my previous machine.