My browsers are: Chrome, Firefox and trying out Opera (Development Channel - due to included free VPN service)
As for OS, I won't bother with vista, then you could free upgrade it to windows 10.
You should be able to follow these steps to install clean windows 10:
http://www.howtogeek.com/224342/how-to-clean-install-windows-10/
Your vista key should work until end of this July
As for flash, Google chrome as flash built-in - so no need to install and update it separately.
Going with Ubuntu Linux is a bit drastic move imo. Here's an idea Alex. If you all you need a bulletproof computer to browse internet, then all you need is a chromebook.
They easy, fast, reliable, virus proof, generally very good battery life .... oh did I mentioned cheap too ?
They start from $150 - they are functional, but nicer ones a bit more expensive:
http://www.amazon.com/Dell-Magnesium-Chromebook-7310-Dual-Core/dp/B015ROJKKO/ref=sr_1_1?s=electronics&ie=UTF8&qid=1463325122&sr=1-1&keywords=dell+chromebook+13