“...stopped the installation by immediately shutting down my PC...”
by saying that, I presume you mean hitting the switch rather than “shutting it down”, as in normal start-menu + shut down mouse clicks?
My PC has a manual power switch in the back (literally on-off) and if I sense my PC is being hijacked, I turn off the power, unplug the internet and reboot then run my anti-virus first thing. It will usually catch it and remove but, just in case, I look to see what the root of folder/program came from and delete it (normally it is in your browser cache if you catch it fast enough).
Before restarting, clean out your browser cache and cookies. Run a regestry cleaner too if you have one.
Oh, and get a second browser. Most of the bad boys are programmed to hit IE so you can bypass it by using another browser.
That’s my advice, anyway, to those reading along.