What is in those unwanted Windows? (Or are they tabs?) Websites, ads, blank . . . . .

I have Firefox, Google Chrome, and IE 11. All of my browsers home page is set to "Yahoo". When I launch any of the browsers, they launch about 5 open windows. All the windows that open up, have "Yahoo" as their home page. If I do search, it also sometimes opens additional tabs within that window. When I launch File Explorer, it also opens multiple windows. It seems like an issue is with most of the apps.

I have tried several different antivirus programs. None of them seem to find any issues. I also reimaged my PC 4 times. It seems like after a while this issue comes back.

Don't know what to do anymore.

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

I tried most of those programs. Nothing helped.

Is there anything else I can do?

Thank you.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia