Sounds like there is something squiffy with your fonts.
It could be.....
Your email viewer is set to use a font that isn't working as default in your message viewing window. When you go for a reply or forward it uses a different font which is working.
You viewer is set to use different colours depending what it is doing and you have white on a white back ground set for reading.
An easy way to check would be to create a new account on the machine then log into that one and see if that solves it.
If it does I am will to bet kids/grand kids will be to blame.
It will be a local problem and nothing that your ISP will be able to help with especially the numb nuts on first line support.