I have a new PC with built in sound. If I install a sound card will the system automatically use this as the default or will I have to manually change it to now ignore the build in sound? P4 2.4ghz, 512mb, WXP.
The system will generally select an add-in card over the onboard sound as your primary sound device. You can disable it in the BIOS to avoid any potential problems, though.
It is possible to have both working. You just tell windows in the audio control panel which card you want the sound directed to. At least, it's much better supported in Windows XP and Windows 98 with a recent version of DirectX.
Generally you'd want to disable onboard sound in the BIOS if you have an older motherboard (rule-of-thumb: not made in the last 18 months).