This player has 2 different USB modes, MSC and MTP. Most likely, it's set to "Auto Detect" and it's showing up as MTP on your XP computer and MSC on your 2000 computer. This is why you see different folders on each computer.
I would recommend switching the player to MSC mode (Menu > Settings > USB > MSC), then it will be consistent. Once this is done, create a Music folder and use that to store all your songs. The player should recognize all the songs in this folder that you create.
If you still have songs in MTP mode, then take those out (using the XP computer) before you begin using MSC mode.
At my home computer (Windows XP) I see the player as a removable drive and can store music in the "Music" folder, I can also then listen to my music with the ear phones. All good there.
I brought the player to the office to transfer some mp3s I had in my Windows 2000 computer and I can also see the player as a removable driver, but the "Music" folder is nowhere to be found, instead there is an "Audible" folder where I can drag the music, but later on when I try to play it, that music and that folder are not showing on the display of the mp3.
Please tell me what's going on. I cannot find any help on the sandisk website