Today you can get a decent sound right out of the box.
However, changing a couple of settings using the sound panel of your PC can make a difference.
In general any kind of DSP will alter the bits.
This is considered bad, there is a strong purist tendency among audiophiles and bit perfect is one of the holy grails.
However, DSP can have benefits like active crossover, room- and placements corrections, etc.


All operating systems are capable of sample rate conversion. This in general won’t improve the sound, as it is not a trivial job to write a good one.
The K-mixer in XP is an infamous example.

Here you will find a couple of tips and links to configure XP, Vista, Win7 and OSX.

If you use a USB DAC there are some simple settings you should check.