Your problem most likely lies in what you are using to open the files.
You should be using a 'plain text editor' in the first instance. However I appreciate that excel may present a CSV in a more readable format.
Normally when you open the file you will likely be presented with a dialogue asking how to handle the file, in this you will have the option of setting the field separators, and encoding.
Your problem is going to arise in saving the file back to CSV, microsoft with try and second guess you, and will attempt to adjust the encoding, and field separators to something more modern (read wrong for Odoo).
In the first instance of using Odoo I was rather surprised that they used UTF-8, I'm more used to seeing one of the IS0-8859-X forms, but that is probably just me.
Your best bet may be to get a version of libre office or Open Office, the spreadsheets in them are more friendly to saving the file to the format that you started with.
I don't know if it exists in windows, but on Linux you can often set a specific encoding for specific file types.
David