Probably you have to disable the built-in sound card in order to use the PCI. That's what was done on my current computer (I say done, because I had the shop that was building it disable the onboard sound card & video card).
There is lots of advice about building a computer, here and all around the net, and there are advocates of every motherboard, processor, and brand of RAM you can imagine, but the basics are: a fast mb/processor (these days, P4 or equivalent), at least 512MB of RAM, and 2 7200 RPM hard drives (one for operating, one for audio). My local shop built me one that's been very stable for $700, minus the case. And I recommend XP, for the simple reason that all the new software is written to run on it. My wife & I have 3 XPs, all running trouble-free. Disable the onboard sound and video cards (they eat up horsepower) and install the card you have and a midrange video (assuming you're not a hardcore gamer or anything) and KEEP EVERYTHING ELSE OFF THE COMPUTER! I have 2 computers, the one I'm typing on which I also use for graphics, and my recording setup, and the recording one stays in the studio with recording software and CD burning software running and that's about it. [My son-in-law is a computer geek, and he constantly brings me software to load on the recorder, like Shockwave, or the latest mp3 whatnot and I have to bar the door to keep him away from my baby.]