ASUS battery problem...

Hello, I have an Eee ASUS notebook(?) I believe...
Anyway, I was just on it uploading songs to my phone, when all of a sudden it shut down on me.
I left it for a good while thinking it had just over-heated, but when I went to turn it on at the normal time I thought it would take / has taken before, to cool down and get ready to start up again, it wouldn't.
I put in the power cord and it turned on. 'Bout time.
So, it loaded all up and I saw that the battery said I had practically no charge, yet before it shut down it had about half left. I decided to test the theory on whether or not it would turn off if I took the cord out, as a friend of mine said her computer would not work at all without the power cord in 24/7.
I tested the theory and the conclusion was; it shut off immediately.
My friend said when she told me that previous fact, that she needed to go and buy a new battery, and I've been told you should replace lap top/ notebook [whatever] batteries every 6 months...But they are like 600 bucks, aren't they? Anywho, I was just wondering if there is some way to fix this, or if I indeed am left to the fate of having to buy a new battery, or worse, a new computer... :/
Thanks.