Just get a router, seriously. Doesn't even need to be a particularly good router if all you care about is being able to do basic web browsing.
Ad-hoc networks are intended for specific, limited, uses, and this isn't one of them. Getting a router would be far quicker and easier than trying to teach you enough networking basics to be able to set this sort of thing up on your own. Especially since after you learned enough networking basics to set this up on your own, you'd agree that a router is a far better solution.
I am trying to set up an ad hoc network using my vista pc so my girlfriend can use her mac with os x to access the internet.
I have an ethernet connection and am using a pppoe connection.
I used this tutorial to set it up and it works fine. I can access the ad hoc network with my iphone.