i don't take credit for this, it is a copy and past from here: http://www.bbc.co.uk/worldservice/learningenglish/youmeus/learnit/learnitv264.shtml
I - me Personal pronouns in English have one form (I, he, she, we, they) when they are used as the subject of a sentence and another form (me, him, her, us, them) when they are used as the object of a verb or follow a preposition (with me, after us, etc). This applies to all personal pronouns, as listed above, except you and it which remain the same in both subject and object forms:
* We gave them some chocolates and they gave us some wine.
* I'll lend you my flared skirt if you'll lend me your blue denims.
* Can you see Paul and Julie? You can't see me, but I'm standing behind him and beside her in the photo.
So whether you say you and I or you and me in co-ordinate phrases depends on whether they function as subjects or objects in the sentence:
* You and I should go and speak to Trevor about this matter.
* Trevor has indicated that he wants to interview you and me.
Note that in colloquial informal British English, people often use you and me as subjects, even though it is known to be incorrect. This has led to an assumption that you and me can never be correct and people (even the Queen perhaps) then sometimes use you and I as objects instead of the correct form you and me.But for your own convenience, keep a clear distinction between them as the same rule applies to other personal pronouns, i.e it's she and I when they are the subject of the clause and her and me when they are the object:
* Do you know Geoffrey? Well, he and I are going to Stamford Bridge to watch Chelsea on Saturday.
* They wouldn't listen to her or me when I said we couldn't go with them.