My pc is a custom built and its specs are listed on the specs below my picture on the left.
Except Operating System.
Assuming Windows 7.
1. Right click speaker on task bar
2. Choose "Recording Devices"
3. Choose Auxilary or Line-in; Optical Input may in fact show up as a separate entry, but only when it's plugged in.
To find out which one you want, have the PS3 play sound; it should show up in the small graphic leveler to the side of most entries.
4. once you've found it, select it and choose properties, or right-click and choose properties. Then, on the dialog, choose the "Listen" tab, and check off "Listen to this device".