Did a quick search and the Inspiron seems to be a desktop PC (I get mixed up with Dell laptops and desktops easily) and I don't think an actual microphone will be built in to the compuyer, or Vista (don't really think your son's friend knows what he's talking about when he says "it's built in to Vista") unless he means software, which for that matter you will still need to buy a microphone seperatley.
You will most likely be able to pick a decent one up pretty cheap, and just plug it in to the pink microphone slot on the back of the PC. It should work instantly, and your son should be able to record with it.
If not, then just have a look at the microphone options in control panel by going to "Sound>Click microphone and go to properties bottom right>levels" and just make sure the microphone levels are not muted or anything.