My audio output device is not install

It's the speakers - unplug them, reboot and plug them again. A window should appear asking you what have you plugged, select speakers.

though itz unclear to me of wat os u r using..
assuming that u r using win xp pro sp2..n directx installed..
check if Audio device is enabled in bios..

howevr, download microsoft kb888111 update n install it..u need to re-install or update the device driver (may b manually)..logon to internet n seek the package/audio driver..along wid the company, there will b a list of such site n driver download links if u search by the model name on a search engine like google..
download them onto a location; if the driver is in a zip format, unzip it,if itz in .exe form, just run it; if done manually it'll ask for the correct driver and location; relocate the path and a .inf file (wat it'll ask for)....goto the device manager n verify, if installed properly..!!
also download n install k-lite mega codec pack....

if not fixed try using a standby pci snd card (in such case it'll b a confirmed case of ur system board is likely to b defective)..
but always better to consult ur system admin/vendor....

You can download the audio driver for this computer so you can make the sound working .....

http://support.dell.com/support/downloads/devices.aspx?c=us&l=en&s=gen&SystemID=DIM_PNT_P4_CEL_2400&os=WW1&osl=EN