Redirecting addon that I can't detect or figure out how to uninstall

I'm sorry if this seems like a total newbie question but I can't figure out if this is a Windows 10 thing or something that's gotten under my radar and is a bit more sinister - everytime I start up a browser, Firefox or Chrome, I get this page Dropbox - Screenshot 2015-12-23 19.44.32.png and I don't know why and it's really annoying. I installed avast to do a virus scan (it found nothing and also broke my internet so I have since had to uninstall it) and I can't seem to find anything out of the ordinary in my add-ons/extensions etc (not to mention I have not installed anything like that for Chrome since I don't use it).

Anyone know what I can do? I also looked at my installed programs and couldn't find anything I didn't recognise.

simrick said:

Hi.
I don't understand why Avast would break your internet connection. What AV are you currently using?

Try running RKILL. This will temporarily stop any suspicious processes; everything it does is undone by a reboot.
When it's finished, it will put a txt file on your desktop. Copy the contents and paste them here.
Note: If any of the following scans require you to reboot, please do so, and then immediately run RKILL again before performing the next scan.

Uninstall any and all toolbars.

Run TDSSKiller, and let us know if it finds anything.

Run Malwarebytes Antimalware Free

Run JRT (Junkware Removal Tool)

Reset your browsers. Reset Edge (even if you aren't using it).

Run ADWCleaner: please make sure everything you are deleting is unwanted. Sometimes it flags things that you may want to keep. If you're not sure, run a "scan" only, and post the log here for us to have a look at.

Run SuperAntiSpyware Free

Reboot and see if the problem has resolved.

thank you so much, running malwarebytes anti-malware fixed my problem!