Under review
This is simply not true. Our installer does not install any 3rd party software...and we are proud we are providing free software without bundles.. Download our software from our webpages is safest way..... If you found some installer which act in different way, then please send me link where can I download it. I know about download.com server which bundles any software in own installer and we can't do anything with it.
I've been a user of System Explorer for several years now (since v1.2 or so) and if memory serves correctly, even the earliest versions of System Explorer's installer were NEVER bundled with adware. My guess is that you downloaded SE from a mirror other than the official systemexplorer.net website (i.e CNET), and those sites sometimes bundle their own adware/spyware into a downloader software for that website. Again, CNET is a perfect example of a download website that does just this.

EDIT: It seems Mistercz typed up a response while I was typing up mine. As you can see—straight from the mouth of the developer himself—SE doesn't include adware in its installer...      Nevertheless, I hope you can figure out the culprit, as I think I can speak for EVERY Internet user when I say that adware is utterly unpleasant.