Yahoo! email transliteration from English to Thai

You have listed the three reasons, so it's good that you are aware of them... Let's go through them:

1. Hacking

   You handled this just fine by quickly changing your password.

2. Spoofing

   The SMTP protocol (the connection used to send mails) allows anyone to specify the address the mail originates from. So for instance, I could send a mail from any Yahoo address without knowing any password, it's possible but it's illegal...

   Most of the times, you can't do much against spam but if it's keep happening you can at least try to identify where the mails are coming from. You can do this by requesting someone who received your mail to check the mail headers and send you the data that is in there.

   This allows you to inspect the IP/host it comes from and inform their ISP about the spam.

3. Malware explosion

   You handled this just fine by verifying the computer is clean.

If you pay for the POP3 service from Yahoo, you can use the option in Gmail to import all your mail from your Yahoo account, either copying or moving the emails. Gmail will use POP3 to access the server and download all existing emails in the background (You don't have to have Gmail open in your browser).

I'm not terriblely familiar with Yahoo. You can check to see if there's an option to "redirect" all emails to another address. That will redirect (not forward with FW: ) all new emails to the new address.

Here's the Gmail documentation on importing email from another account.