Did you do what I said? There are
two steps. You can't just uncheck it in MC's options and expect it to fix itself.
When you check one of the file types in MC's options, it essentially just says "MC, take over the file associations for this file type every time you start up". When you uncheck it, MC will no longer take over the file type association anymore, but if you want it to point at a different application you have to fix it yourself. MC doesn't magically know what program you want to use for those files, it just knows that you don't want it to automatically "steal" the association anymore.
As I thought I explained above, to fix your XML files, you need to
both (1) uncheck that box (so that MC doesn't steal the association again next time you run it)
and (2) fix the association to point at the right program.
I don't know what the right program is, so I can't tell you what program to use. XML files are general purpose files. They don't "go" to any one program. If you don't know at all how to fix associations, and can't figure it out from my post above,
check this article at Peachpit Press.