Do a search on this board, there is alot of info here.
First off, you have a GoPro Hero HD. The older models did use .avi as file format, but the new HD models use .mp4 which is not native to anything in Windows less than Windows 7 and will not run in the earlier versions of WMM. The only version of Windows that it WILL work in natively is Windows 7 with the new version of WMM live.
It gets really complicated with XP. You will have to use something else to edit with. I'm trying to use Premier Elements, but it still requires the addition and complicated installation of some additional 3rd party codecs (which Adobe DID help me with installing). However, I'm still having trouble with the file sizes. Haven't figured it out yet. Am thinking about buying Quicktime Pro, as it's supposed to be able to break up or convert .mp4 to other file types without downgrade that you could then edit.
Your other choice is to convert the .mp4 to something else that WMM can edit (.mpeg); however, I have yet to find a converter (free or otherwise) that will convert the file without losing quality (and that's the entire point of having HD, isn't it?)
Good luck