The analogue output from Apple Computers is surprisingly good and an excellent place to start, but PCs seem to sound very much worse and I'm sure this is why people can't believe computers can do audio, despite the fact that most of the music they buy has been made using one.
If you have a PC laptop and don't like the sound, the cheapest and most surprising upgrade is an M-Audio Transit, it's £50 and surprisingly good sounding, very good sounding. However there is a catch and that is the software takes some fathoming out, but it's worth it.
M-Audio is the hardware division of a huge and highly regarded Pro Audio company called Pro-Tools, so their credentials are impeccable.
Ash