Re: I need a bit of advice!!!!!
If you and your gf have complete trust then theres no reason you can't tell her about it. Tell the girl that you are taken already, will be easier for her that way. Be honest with your gf about the email (possibly neglect to tell her who its from straight away, if she asks then tell her) and reassure her that you weren't tempted or interested, that you only have eyes for her etc etc.
That way you don't actually lie to either of them, you don't cheat on either of them and your gf will trust you all the more for opening up to her.
~ The journey of a thousand miles begins with a single step ~
* Nothing in the world has ever been accomplished without passion *