Chrome and Firefox show Alerts, Full-tab Ads and Redirect

Both browsers I have installed, show the same symptoms..

Pop-up alerts claiming you have to download this or that software to "stay safe" which can only be closed by force by closing the browser from the task manager.
Random new-tab Ads,
Random redirects upon goodle search, to random search engines.

So far I've tried:
Look for new unknown plugins intalled in the browsers - don't see any.
To uninstall any suspicious software - didn't find any.
To look for suspicious processes running in the background - didn't find any.
Scanning my computer with Avast and Malwarebytes, they don't find anything.

Any suggestions would be greatly appreciated.
Thanks.

NO LOG
By posting just a description of your problems it is likely that your topic will be passed by and you will not receive the help you're requesting.
We need to know what's running on your computer so that we can provide the appropriate instructions.

May I draw your attention to THIS topic, which you should have read, that states what we need you to post, so we can help you.

If you still need help, please start a new thread an include your FRST logs:

• FRST.txt.
• Addition.txt.
• Details of the problems you're experiencing.

If for any reason you can't run FRST, please let us know in your post.

This topic will now be closed.