PC speakers are not really made for mixing or mastering. They are made to make things sound good on a small budget for the mass market. This means that they probably have a very tailored frequency response, often with a typical bump in the bass drum region to make small speakers sound larger. As such they are very good when playing computer games and when listening to finished recordings. As tools for serious music production they are rather limited though.
What you really want to have when mixing and mastering music is tools that you can rely on to convey the bitter truth. A bad recording should sound bad, allowing you to fix it as good as possible. The effect of any fix should be easy to hear. There is a large market out there with "studio monitors" supposedly giving you that. They start at a price above the computer speaker market and goes into sky high. Again quality does cost. But there really is a large difference. They really should be connected through specialized sound card as the built-in sound card will not give you the full picture. Add to this that in order to be able to hear low frequency accurately you absolutely need to treat your room acoustically. It might however kill the joy you have in listening as you will start hearing a lot of "bad things" instead of the pure joy of music. Sorry, there is no free lunch.
// Gunnar