Vista - "no audio output device installed"

Hiya, so my problem is that i have no sound, haven't had for a while now.
When i hover over the speaker icon it says "no audio output device installed".
I've tried going to device manager and beside where it says "Realtek AC'97 Audio" is a yellow triangle with an exclamation mark inside. I have tried clicking update driver software and scan for hardware changes.
I have been googling this problem for ages to try and find a solution but anything i can find doesn't work. If anybody could give me any suggestions on what might be wrong and how to fix it i'd really appreciate it.

I don't know much about the computer, it was given to me and i'm not much of a computer wiz. But if you need to know anything i will try to answer.

Thanks.

Hi Jodie, a quick Google search gave me these drivers for you. Try each one and see which one works:
Realtek AC97 audio driver for Windows Vista download - Vista AC97 driver 6285
or
Realtek AC'97 Driver (Windows Vista) - Free software downloads and reviews - CNET Download.com
or
Realtek AC97 Audio Codecs Vista A4.04 Driver for Windows Vista Free Download - Softpedia