Indeed, I had problems with some crazy software, but weird things started happening with v.18 of Google Chrome.
I use Avast, as Anti Virus, and I found out that several people, using Avast, ran into problems, when it updated from v.6 to v.7. People who never had Chrome found themselves with Chrome installed. It was not necessary to enter a password to look at the Settings of Avast, prior to v.7. I found that I could not even look at my settings, and I had no password. My existing Google Chrome profile was messed up, also.
Finally, I got a password, and was able to take a look at my Avast settings. Sure enough, there was a new box, "do not make Google Chrome my default browser", which was unchecked. Google Chrome had not become my overall default browser, but only for Avast. Some websites will open Internet Explorer, and, later, Seamonkey, but Avast did not open Seamonkey, until I check that box.
I read that a lot of Avast users went to another program. This is another case when developer think that they know best.
This issue came up in February 2012, but, I did not have full understanding, till now.
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:13.0) Gecko/20120615 Firefox/13.0.1 SeaMonkey/2.10.1