DVD decoder with 5.1

Hello ALL,

OK, I need some help from you Guru's out there! Here is what I have:
Sony VAIO VGC RA-710G, P4 3.2Ghz, 4GB Ram, and VISTA Ultimate.
PC Sound System - BOSE Companion 5 (Connects to PC via USB 2.0)

Question, I have been reading a lot about the DVD decoder with 5.1, more
importantly, the need for one, since the BOSE uses the 5.1. So before I go
out and buy something I may already have I come here. Does VISTA Ultimate
have a DVD decoder with 5.1 incorporated into the OS?

IF YES, GREAT, how do I access it, through the SOUND Control Panel? If the
answer is yes to that, no need to go further, I know what to do.

IF NO, PROBLEM, how do I get it and which is the best one to get? More
importantly, do I need it?

Thanks! Joey