1. 3gp is suported on windows 7 please see this
2. It was not supported on XP
It is not the movie maker that is different per se it is the attendant conversion software
3. On XP you will have to search the web to find a free or if you do not mind purchasing a paid conversion program
I would be hesitant to recommend. as I have no experience of them.
One example and I am not recommending it, only offering it to you for your consideration
4. I hope you understand, what follows, but it was remiss of me not to mention this on my first reply.
Whilst it is permitted to record at some concerts, it is not at many. I understand it generally depends on the "rights" to the music being performed.
To record video and audio at many concerts is not allowed and Geeks to Go does not support anything that contravenes any license, copyright, performing arts conditions etc.
Therefore the advice is offered on the understanding that we do not know the conditions attached to recording at the concert you attended and should only be used when you have satisfied yourself that you were so entitled.