I'd not get too, too strung out on the notion that a laptop will always outperform a desktop, sonically speaking.
I have a new toshiba using Vista sp2B. and ran out USB to the BC D3... and raved about the sound too.
then I began to compare, change things about with my desktops, and via my aforementioned notes have decidedly a better sonic presentation through it rather than with the laptop.
Now I should also say this about that... the Dimension 4600 is not stock. The video and ram are both upgraded as is the power sup. it rests upon a damped Sound Anchor rack, with iso devices beneath it, and on it's side too BTW. it's fed by a VD Power III cord whcih is supplied from a PS Audio Duet that is fed from a 20 Dedicated outlet by a Shunyata Python VX.
From it's video card I feed that signal into my Onkyo receiver via HDMI. I can also 'laptop into it' by 3rd party software... but hardly ever do.
I usually just setup some lengthy playlists of varying genres, use the Projector to get started, a lengthy USB cable and hub to connect the keyboard and mouse too... and once into the media player, I just shut down all the video gear and get to getting, with the music.
One consideration in my affair too is the length of the USB cabling IN use. It's about 15 ft long... either way... desktop or laptop!... as my audio system resides in an adjacent room to my dedicated listening/viewing room.
I have personally seen - heard, diffs in USB cables too... by virtue of their lengths primarily. the longer seems least best, generally. Under 10 ft, it matters less, and under 6 ft I don't think it matters at all who made the cable.
so there are any number of additional factors apart from the primary devices at play here too. Consequently, the often apt A'gon caveat is still true here too... "Everthing matters" .... how much it matters, though, is up to you.