You don't say where the message appears. If it's in your email it's
somebody trying to hijack your computer and you're right, you should
not download anything. If it's a popup in the middle of your screen,
there are two possibilities (really only one): someone else is already
controlling your computer. It could be either a completely
unauthorized invasion, in which case you need expert help to remove
the malware, or it could be one of your authorized applications trying
to be helpful without asking whether you really want help, such as
the Software Manager that sometimes comes with Installshield.
To remove or disable Software Manager without going to untrusted
sites, search for "installshield" in Google or another search engine.
The primary result site is one on
flexerasoftware.com; go to that one.
From there choose Support and then Consumers with Installation Issues.
There are two ways to do that; both get you to
consumer.installshield.com. Under The Update Service, choose FAQs, and
from there you can browse for information. Beware that some of the
links take you to unexpected information, so try them all, including
the links in fine print at the top of the page.
If it's the Installshield update service, the basic idea is that the
customers of Installshield are software vendors, and Installshield
helps them "maximize their value" by getting more money from their
customers, like you. Unfortunately, some of Installshield's customers
don't use the update service correctly, and the result is annoyance to
the users. Flexera, the company that now owns Installshield, is aware
of this, and they want to keep their customers' customers (like you)
happy.