I don't know what it is, but I googled it and found ways to remove it. The URL is below.

Here is a link that might be useful: istart 123

It is a browser hijacker. Go to the link Emma provided and scroll down to ADWcleaner and use that.

I suspect you downloaded something you wanted but failed to read each screen during the install. These days it is vital that each screen is closely read and check marks added or removed as needed to do a clean install. ALWAYS select a custom install so you actually see each screen and have the opportunity to exclude all the unwanted PUPs (potentially unwanted programs)

If an adware cleaner does not get rid of it then download Revo Uninstaller it will uninstall anything on the list that comes up.

thank you will try Reno uninstaller

I tried revo uninstaller and istart 123 is still there...

Did you try ADWcleaner, linked in Emma's post?

If you are using IE as your browser then do a reset, go to tools when browser is open, then internet options, then advanced tab on top, and let it reset.