Sounds like your neighbour has formatted the hard drive and reinstalled windows. And has forgotten to install the motherboard drivers if you have onboard sound as most of us have - hence no sound.
See if you have the motherboard disk and reinstall all the drivers and that should cure it.
If you don't have a motherboard disk then you will need to find out the make and model of your motherboard, goto the manufacturers website and download them from there.
To find out what motherboard you have, if you don't know, you can use either Belarc or SIW click here click here