By using an appropriate viewer.

LimeWire has built in support for playing MP3 files (I don't use it and I've heard that it isn't very good). For everything else you'll need to use third party programs.

What sort of things are you trying to view?

WinAmp is a good audio player (will play most audio types). Windows Media Player should be built in and will play most movies but may need to have additional codecs (there are a very large number of different movie types even within the standard extensions of avi and mpg). I've found the Nimo Codec bundle to be extremely useful. Search for 'Nimo'.

JPG and GIF files can be viewed in internet explorer, although there are much better programs around for the purpose.

Mark