No sound (by 2 users)

To troubleshoot a no sound issue first you must check your device manager to make sure the audio device is installed properly.

To goto the device manager in XP:
Right click my computer>Properties > Hardware Tab > Device manager.

Look for your audio device in the list under "Sound, Video and game controllers". If it has a exclamation mark or question mark beside it you will need to download the most recent drivers from the manufacturer's website.

Next step is to goto
start>programs>accessories>entertainment>volume control
make sure that Wave nor Master Volume is not muted, and both are turned up.

Third step is to check for onboard volume control, there might be volume controls on the system hardware itself.

If you say that external speakers work and not the built in speakers, then the headphone jack that you plug the speakers into may be defective. As it is always detecting something plugged into it and never turns the internal speakers on.