your computer is going to process sound at 32 bit (windows) and that is even higher than it is recorded (a cd can be nothing other than 16 bit). so my point is, the computer/software isn't what is lacking, it's the hardware. you have a chain that goes
sound treating the room (if you wanna get real crazy)
^
speakers
^
amplifier
^
preamp/equalizer (the sound card)
^
d/a convertors (digital to analog, that's the cound card)
^
computer
upgrading all of those things, or at least a few, are the only thing that will give you better sound. a chain is only as good as its weakest link... there is no way around it...
you don't have to spend an arm and a leg either, i would start with getting a new sound card. if you have an external amplifier for your speakers, get a "usb audio interface" which are $50-$100 at any music store... otherwise, if your speakers are self-powered, get a new internal soundcard...