No audio output device is installed acer

hello, im jim. I have an acer laptop: acer 4520, windows 7. my laptop is probably old to have a windows 7 as its os. well let me tell you the problem.
when I installed windows 7 to my laptop, with all my drivers installed too, there was no problem. sound and everything was ok. until such time that when I turned on my laptop, I hear no sounds anymore. I started to solve this problem by re-installing the sound driver(realtek audio high definition). after re-installing it, still the same problem occured saying "no audio output device is installed" on the sound icon on the taskbar.
a day after I tried to turned on my laptop, and whooosoo it has its sound again!. happy for that until I restarted it,, booooom.. again its lost. the same problem appeared. its like an "on and off" because it comes and dissappears! T_T
i had a hard time formatting, installing back windows 7(did it almost 5 times!) but the problem would still be there.
i wish u could help me with this,, ill be very glad if u could share ideas
sorry for my bad english ^^V