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you have to get into the BIOS...
im not sure what it is for a Dell PC, but normally to get into the BIOS you hit Esc, or one of the F# keys (1-8) right after the computer boots up, and before it loads windows. thats when you can get into the BIOS
now when in the BIOS, there will be a number of options (all Bios menus are somwhat different, if not from the same company) do not go around switching other values if you dont know what you are doing, only look for the option that has something to to with onboard sound, it will give you the option to enable/disable it.
you may have to fish througha few sub menus to find it
but what i would do is, look on the Dell site to see what key you have to press before windows starts, and what menu the sound option is under
peace
LB