Altec Lansing multimedia speaker system

I have an Altec Lansing 5.1 surround speaker system model ADA 995 purchased with a Dimension 8200 desktop a few years back.  I want to use this system with a new Inspiron desktop I am considering purchasing, but I think there is a compatibility issue.  The old speaker system has a 3-wire cord out of the subwoofer, and the new Inspiron (I think) has only one speaker jack thingy.  Is there a speaker adaptor or something that might help me make the old speaker system work with a new computer?  Thanks in advance.

 

Sorry guys - I am such a dumb girl sometimes.  All thingys will fit the sound card:

 

http://www.dellcommunity.com/supportforums/board/message?board.id=xps_desk_audio&message.id=44&query.id=65018#M44

 

and

 

http://support.dell.com/support/edocs/systems/insp530s/EN/OM/setting.htm#wp1120549