If you simply want to join videos together then you should know that it has very little to do with aspect ratios. What you have is what you have.
Now, if you are trying to re-encode or "transcode" videos into a different format - even if it's to adjust/change the aspect ratio - then you probably do need to worry about ratios as well as codecs, sampling rates, file containers and so on.
But you asked for suggestions on joining videos (and possibly a better editor). And for that I would suggest trying Avidemux. I've not had a lot of luck with Avidemux joining videos but it does do it. And once you get used to using Avidemux you may wonder why you ever wanted to use Movie Maker. After all, free products like Avidemux may be the very reason Microsoft no longer supports Movie Maker.
But if that's not good enough then you may want to try Handbrake instead/too. Handbrake is not so much an editor as it is a transcoder. But you can change aspect ratios as well as video and audio streams. I believe it too can join videos, but I really can't say.
Another really handy video tool to have/use may be the one I use a lot called Format Factory. I use this for transcoding .flv video to .MP4 and even for getting info about various video files (like if it's using H.264 or something else for a video stream). Format Factory also joins (as well as can split) video too. If you get nothing else, then Format Factory may be exactly what you are looking for (minus an editor).
Here are the URL's...
Format Factory is provided by Softonic who you should have no problem finding with DuckDuckGo (or Google). Sorry I can't post the URL since SU only allows 2.
P.S. All 3 of those will work with Windows. Avidemux and Handbrake should also have Mac and Linux versions too.