I have the Logitech X-530 Speakers, which are pretty much the same as the X-540. Here's a link to a diagram I made a while ago showing the lengths of the cords. I'm pretty sure both sets will have the same lengths of wires.
Thank you, but, i think the only difference is the wires all go striaght to the sub with the x 540 and not via the right speaker. I think this becuase ive seen a pic of the back of a 540 sub and all ports are there.
You could get a new sound card, no need to replace the tower. Most onboard sound cards are 5.1 compatable. Each of the 3.5 male jacks on the x-540 stand for a pair of speakers. Green-front pair, Black-rear pair, Orange-center/subwoffer pair. See if you can match the3.5 male pairs with the 3.5 female ports on your tower combination (this may require the owners manuel). Or use matrix mode.