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Those aren't movie files, they are project files. Did you ever save them as movies (File > Save movie)?
A project file is akin to a recipe. It contains the list of files that are in your project, their order, any edit points, transitions, effects, titles, etc. The project refers back to the actual videos and photos that are on your computer, so a project file on its own isn't very useful.
Do you have the original video/photos that were on the XP machine too? You need both the source files *and ***** the project file in order for Movie Maker to create a movie from that. Without it, it would be like having a recipe for a cake, but no eggs, water or flour.
Okay, assuming you do have the source video files, the next issue is the difference in file formats for the project files. Windows XP had Windows Movie Maker 2.1. There is no way to directly read that format in Windows Live Movie Maker.
The best I can suggest is to install Windows Movie Maker 2.6 from Vista. It is based on WMM 2.1 and can read a 2.1 project file. Another option is to install Windows Movie Maker 6.0 which can also read a .mswmm project file from 2.1. The advantage to that is you could then save that project file from WMM 6.0 and it would then be readable in Windows Live Movie Maker. In other words, each version can read a file from the immediately preceding version, but you can't jump a couple generations from 2.1 to the latest version.
Again, I hope you have the original source videos and photos, or else this whole process is moot. Or if you ever saved them as actual movies, that would also be good. In the analogy of a recipe, that would be the equivalent of having the final cake to enjoy.