LAPTOP SPEAKERS WONT WORK

Hello,

i have a compaq presario cq40 laptop. i'm using windows vista black edition 2009. about a few months ago, my laptop speakers wont work but when i plug in the headset, the headset works. i had it checked in the store where i bought it. the people from hp changed my speakers but they think that my system is affecting the sound problem thats why there's still no sound. they asked me to reinstall my windows. i didnt have the time to reformat my laptop so i just let it be.

yesterday when i turned on my laptop, the speakers were working again. so i thought that everything was ok. but now, they're not working again. no sound came out during startup. can anyone help me with this problem?