if its 2 channel analog you can only do dolby pro logic surround (4 channel.. Left, Right, Center, combined Rear). thats a hard limitation of the 2 channel analog signal format. might be the psp can use that mode.
on that particular unit, with its limited inputs, for proper 5.1 (Left, Right, Center, Left Rear, Right Rear, Sub) you will need to use the toslink (optical) input. does your mobo have optical out?
one caveat on optical out from your PC is that games may not give 5.1 over optical, only 2 channel (depends on the drivers and whether they support dolby digital live encoding). however, movies that were encoded with 5.1 will give 5.1 sound.