Windows 7: 5.1 Two Rear Speakers Not Working

Please help me with my problem. My PC configuration looks like this.
Mobo : Intel DX58SO
CPU : Intel i7 950
OS : Windows Vista Ultimate (32 Bit)
Sound : Realtek High Definition Audio

I have a 5.1 Creative LX520 Speaker, I have set the sound output to 5.1 in control panel, but when I test the speakers the two rear speakers does not work. But if I change the setting to 7.1 it works, but the sound from the rear speakers are like distorted. Please help.

Your problem actually helped me figure out why my rear speakers weren't working! You mentioned that the 7.1 setup makes the sound come out of the rear speakers - this made me think that perhaps the rear speakers had instead been mapped as side speakers in the Realtek software.

To fix this, Hover over the coloured ports on the right hand side of the Realtek software until you find the one that says 'Rear Speaker Out (Speakers)'. Double click on it and change it to 'Side Speaker Out (Speakers)'.

Now, Windows and Realtek will both play out of the rear speakers correctly. Hopefully this helps someone in future...