look at this link,
the Audio IO panel of the pc needs to have the relevant outputs on the sound card active ,you cannot get true 5.1 sound without them.
unless you have altec combo which emulates it.
(vista is clever it recognises the connected ports and lets you choose from the options that are detected,.)
one up for vista i suppose.
ive given up with the realtek, i can only get front and rear no sub so replaced it with el cheapo olde sound blaster live which works 1st time with xp.
you can see the ports you need for 5.1 to work properly
black= centre out / sub
green for the rest
also works with creative 5.1 unsuprisingly.
cheers & gl