Look at the back of the computer and compare with the one in the link.
If yours has some that look like the audio jacks in the link then you should have onboard audio.
Next open Device Manager......in XP go to Start>My Computer....right click and select Properties.Then click on the Hardware tab and select Device Manager.
Look for Sound,Video and Game controllers and click on the + sign by the *speaker* icon.
Look for any yellow exclamation marks against any of the listed devices.
Post back with what you find.
Most pcs these days will have onboard sound built into the motherboard.You may need to find drivers for it but shouldn't need to buy a seperate soundcard.
What make/model is the pc?