Each time i got yahoo mail, my yahoo messenger pops up. i don't need it

1. Open Yahoo! Messenger
2. Click Messenger --> Preferences or CTRL+Shift+P
3. Under category click Alerts and Sounds
4. In the "Alert me when" box click "I receive email in Yahoo! Mail"
5. Uncheck the boxes next to "Displaying a dialog box", "Showing an icon in the system tray" and "Showing a message in the bottom right corner of my screen". If you prefer not hear the pow alert as well uncheck "Playing a sound..."
6. Click OK and you're done.

So as not to leave you in the dark, let's also define these three options:
"Displaying a dialog box" is the Windows dialog box that pops up notifying you of new mail. "Showing an icon in the system tray" is the envelope that appears when new mail arrives. "Showing a message in the bottom right corner of my screen" is the box that comes up giving you the subject of the message or how many emails.
Additionally, you can customize all your alerts in a very similar manner: Select the alert you want to change then check or uncheck the boxes as desired. You can even change what sounds you hear.
Hope that helps take some of the aggregation out of your computing experience!\

Start-> Run-> CMD->Msconfig-> Start up tab-> Uncheck Yahoo Messenger-> Apply-> Close-> Restart the computer.