The only "critical" things I would say are, a AntiVirus program and Anti-malware/Spyware in place.
Adobe Flash player is something you'll certainly want. Its used for many things. Especially Video playback, such as YouTube Videos.
Beyond that, there wont really be anything critical for surfing the Web.
Adobes Flash will usually play most everything.
With a few exceptions, such as Netflix, that may use Silverlight.
Silverlight is OK, its from Microsoft. You can read more about it & download the plug in here. http://www.microsoft.com/silverlight/
You may not have a need for this however.
~~Just be careful of anyplace that asks you to install extra codecs or download anything in order to watch a video.
The alrge majority you will run into will be Adobe, and a few may be Silverlight.