You have to download the latest driver for your sound card for it to have the 5.1 support. For instance, I had to download the RealTek driver pack and install it, then double click the red Realtek icon in the bottom right hand corner of my screen, and go to advanced settings and set the preferences for each speaker and each of the 3 input ports on my computers sound card.
If you have a Realtek card then, you want the Realtek High Definition Audio Driver R2.11, you can find it here:http://rapidshare.com/files/173933659/WDM_R211.zip
and the Vista one is here:http://rapidshare.com/files/173930788/Vista_R211.zip
That's for my setup though, if you have a Soundblaster card, it'll be the newest Creative driver you want..
Figure out which one you have from your device manager in the system part of the control panel and then google search for the card brand followed by the word 'driver', and check the download page to make sure it supports your card's model number.