How can I get Gmail to be default email for my Windows Vista PC?
I am using Chrome as default browser. Gmail as default email.

Also;
When I log into Gmail it still shows my yahoo address..and when I compose, it comes from Yahoo address.

why?

> How can I get Gmail to be default email

I believe Gmail Notifier has that functionality:
http://toolbar.google.com/gmail-helper/notifier_windows.html

> and when I compose, it comes from Yahoo address.
That happens on the gmail.com website? That's weird, go to "Settings" -> "Accounts" and make sure that your gmail address is set as default.