There are several reasons you may be getting the "Invalid Password" or "Invalid ID" error message.

Passwords are case sensitive.
Please make sure your CAPS LOCK key is properly set. Do not add the @yahoo.com when signing in. If you have recently requested a new password, please wait at least 24 hours before attempting to sign back in to your mailbox to allow your email account to update.

Difficulty setting your user cookie.
A cookie is a small piece of information stored by your browser to help identify who you are. If your cookie is not being accepted correctly, you will be logged out of your account as soon as you move to a different page. This is what is likely blocking your current ability to log in or stay logged in.

Try shutting all of your browser's windows, and then deleting the cookie files in your browser's directories. Usually these files will be called simply "Cookies." As always, be sure that you have selected the proper files before deleting.