>>>Hi,
>>>
>>>Some of you may have seen my thread about a major memory leak with Outlook 365. I am considering downgrading to Outlook 2010. But, first I wanted to try it. Do you know, if I install Outlook 2010 on the same computer where Outlook 365 reside, if they can co-exist?
>>
>>Historically it wasn't possible to have 2 different versions of Outlook running side-by-side. The so-called "click to run" installation of Office 365 apparently relaxes that somewhat - with problems: https://support.microsoft.com/en-us/kb/2782408 . However, if you've already done a full install of Outlook on your local machine I don't think you can successfully install Outlook 2010 side-by-side.
>
>Do you think I can successfully completely uninstall Office 365?

I don't see why not.

If you are, or suspect you are having problems with antivirus I'd do a complete uninstall of that first (using a clearing tool like AvastClear). Then, while you're temporarily running no antivirus at all, see how Outlook functions.