Error "another installer is currently running, which must be closed before installing this product."

I cannot download because "another installer is currently running, which must be closed before installing this product."  But I don't think another verison IS running.  First I tried this from my own user account, and was not able to proceed without logging in as an administrator. Now I am logged in to the admin account, and I tried to direct the download to my own user account. The first time I got this warning, I logged back in to my account and verified that no download application is open there... but I have the same error:  "another installer is currently running, which must be closed before installing this product."