It sounds like a browser hijacker, I've seen a few of them recently. Thankfully, these ones tend to go away if you reboot - previous vulnerabilities in web browsers meant they were much harder to get rid of and would come back after a reboot.
Scan for viruses - which you did - is a good thing. Wouldn't hurt to run malwarebytes as well.
If you've not already got it, go here
https://www.malwarebytes.org/, and download the free version, then run the installer. Near the end of the installation, untick the "enable free trial" option - this will then give you the free free version, which is good enough.
Run it over the whole disk and see if it picks anything up.
Also worth making sure that flash is up to date, in theory Chrome and IE should have updated themselves, but go here to check which version you're running and which is the latest :
http://www.adobe.com/uk/software/flash/about/If you're not running the latest version, click the link to the download center.
Note that Chrome has it's own version of flash installed, click the three lines on top the right hand side of Chrome then choose "about Google Chrome" and it will check if it's the latest version - this will update Flash in the browser as well.
Windows/Microsoft update has been sending Flash updates for IE today as well.
Ian
