With such low powered computers, forget about mining. There was a time when you could have made some coins from CPU mining but now it's really in the hands of the people who have invested in it (think big racks of high end graphics cards).
But welcome to bitcoin! You've found one of the best currencies around
To get started trading with real people in personal transactions, join the room #bitcoin-otc on the irc network irc.freenode.net (http://webchat.freenode.net/
For smaller amounts with fees but quick access to coins, http://coinpal.ndrix.com/
For larger amounts head toward http://mtgox.com/
So, it's not the free money that you thought it was (though you certainly can play the market and potentially make quite a bit), but it's still a damn good system!