Since you are running Windows 2000, you may have a harder time finding the trojan as I think some of the better trojan elimination tools like Microsoft's free Security Essentials need a more advanced OS (double check, though).

If you have been using web-based e-mail for some time and it's sending out e-mails to everyone in your address book, you will probably start seeing a bunch of undeliverable messages in your inbox because of old e-mail addresses that no longer work. If that's the case, then some keystroke trojan got your password and sent it back to its host.

If you are using the web version of Yahoo mail, get to another clean computer and change your password. The e-mailings will then stop. I went through this a while back with the blank subject line and a link to viagra sites. I can't remember the name of the trojan but Microsoft's Security Essentials was the only one that found it and got rid of it.

I was probably spared more grief because I only surf the web in a user mode, not as an administrator.

Kevin