Cannot open Windows Live Mail : Repair the Installation
Repairing the Windows Live Mail installation won't hurt anything, but it might help if there's a problem with one or more of the Windows Live Mail program files. A repair will check that all the program files are present and correct, and (where necessary) are properly registered.
Accordingly, this type of repair will put right any error in the file associations, by re-associating the .eml file type with the Windows Live Mail program in the Windows registry.
To do this repair, close all running programs, then press the Windows key + the R key simultaneously, then type WLARP in the box that opens, then press 'Enter'. In the window that then opens, click on "Repair all Windows Essentials programs".
When it finishes, restart the computer. Then launch Windows Live Mail, and test it to find out whether the fault is fixed.
The trick to getting this fix to "take", so that it doesn't keep reoccurring, is to (a) close all open programs first; then do the procedure; then (b) immediately restart the computer. You are making changes in the Windows registry in the procedure, but these don't get recognised by Windows until the registry is reloaded on the next start-up.
If you are intermittently running some incompatible software, i.e. any program that tampers with the e-mail settings (for example, CCleaner or Norton 360), or another e-mail program, you will still get occasional problems where you have to re-do this fix.
The easiest solution is to uninstall any other e-mail software on your machine, and the incompatible software, so that those programs can't make the type of changes which will interfere with Windows Live Mail's settings.
When a problem causes the WLM program to stop working, and Windows closes the WLM program, it will notify you, after a few moments of checking, as to whether a solution is available. If Windows hasn't notified you within a couple of minutes, chances are it never will, which means it can't fix the error. That means you have to. Doing a repair is the obvious first step for you.