iStart123 is a virus that gets secretly attached to your web browsers and it is virtually impossible to remove. I know I had this happen to me today when I opened up Firefox. From what I've read, it comes attached to updates made by Java or Adobe Flash, so be aware of updating Java or Adobe Flash. I had spent just about the entire day today trying to figure out how to get rid of iStart123 and finally found the solution.
This is a 4 step process and you will need to do each one consecutively.
First download AdwCleaner (Free)http://www.bleepingcomputer.com/download/adwcleaner/...
and run it
second download JRT it is a junkware removal tool (Free)http://www.bleepingcomputer.com/download/junkware-removal-tool/...
third download malwarebytes this one has a free (trial) version or a premium versionhttps://www.malwarebytes.org/
(once downloading and installing, if it doesn't do an auto update check-run one, then run a full Threat Scan. After the scan is done, go to Settings>Detection & Protection > non-malware > change the settings in the PUP and PUM to Treat as Malware if they aren't already set this way. Scan again.)
final step is to do a free online scan (This scan will ask you if you want to scan for pups tick YES)http://www.eset.com/us/online-scanner/
I am also keeping the adwCleaner and JRT removal tool on my desktop and planning on doing a once a month Cleaning of my whole system.