you can tell by checking what the colored jacks are connected to, a card or your motherboard

this is what a card might look like

fig 1

if this is similiar to what you have get the numbers and any other info of the chipset. the arrows are pointing to the chipsets in fig 1

If your sound is integrated on the motherbaoard, locate the chipset and get the numbers off of it.

the chip fig 2 and fig 3(circled in orange) is usually located in close proximity to the CD IN and AUX IN fig 2 and fig 3(circled in green)

fig 2

fig 3

usually windows will locate and install the drivers for you automatically but since your computer is older and you cannot use windows update we must resort to these methods. They are a pain in the butt but presistance can pay off.

now that you hopefully have found the numbers off the chipset, take those numbers and google them or any other info you found on the chip. hopefully you will be able to find the drivers that you need.

If you need help post up your findings here. A picture would be great.

take a pic ... then upload the pic to imageshack.us then post up the "direct link" to the pic here I'll see what I can find for you if you require assistance.