Encrypted forwards and replys!

I've had a problem when I forward or reply to an e-mail. When the e-mail comes to my inbox text is clear and concise. When I fwd. or reply(it does on either one), the text looks like it is encrypted. I had started using Netscape prior to this problem. So I switched back to Firefox, the problem was gone for about one week, then it came back. I switched to Internet Explorer, the problem was still there. I contacted my ISP, they had me do a restart,which I'd already done more thn once, they checked some other things, by the way when I contacted them the problem had disappeared. We tried to replicate the problem by them sending me an e-mail, then I replied and of course it worked fine. Then we did it again and I fwd it and it worked just fine.

Does anyone have any ideas? One thought, could my computer have a virus?

I didn't think to ask, do you all try to troubleshoot problems that people send to you in an e-mail.

Thanks