Thanks very much for the welcome and the advice.
I have never knowingly installed anything like you describe, let alone even heard of Returnil. I've been using this pc for years (yeah, yeah, DUH, given my operating system) and I've never had a problem like this before with other free, reliable and mainstream security software. I cannot honestly say I am absolutely CERTAIN, but I only noticed this when I tried to update to 2.26. In other words, I believe I previously used versions in-between without this problem.
THREE THINGS that might be important:
1. I recently switched from dial-up (Earthlink) to cable (Comcast) access.
2. I recently switched to Mozilla Firefox as a browser and, because it's my home machine and I use it mostly for basic stuff (local weather forecasts, personal e-mail, shopping) I do use My Yahoo as a homepage from which I surf.
3. The CCleaner directs me through the update installation process using Internet Explorer, which otherwise I do not use.
Could any/all of the above be the problem?