I agree with mrz re teaching letter names first, I'm an ex literacy co-ordinator.
I wasn't that keen on what I saw of Headsprout(no offense either),have to say my very literate 5 year olds would die of boredom,it's very dry and some of the text is shockingly tedious.
To the op masses of quality picture books are a must,if you really want to you could teach the sounds(correctly of course)but I wouldn't clutter her brain with much else at the moment.
My 3 are very literate. Most of this is due to being immersed in literature from a young age which led to them becoming book addicts. Dd4 sits up in bed doing pretend reading night after night,she memorises a lot,uses book language and is starting to guess at words,point out sounds etc. She wouldn't do this if she didn't adore books. Her brothers 5 are very advanced readers and were very like her at this age.
Go to the library,find authors you both love, look at Red House and Book People(good offers),read to her at every opportunity,go to book shops and read there.
If you do all this now it will give her a fantastic grounding which will give her a headstart when she starts school.