some future software may not work on your pc if you don't have service pack 2 installed. although im not sure if any do now.

you can download and install a network addition of sp1 and sp2, which means you can download and burn them to disc using your internet pc, then install them on your music pc.

some good xp tweaks here http://www.tweakxp.com/ and here
http://www.tweakhound.com/xp/xptweaks/supertweaks1.htm

i've always used the tips on tweakhound.

don't install anything you're not using for music, so no firewalls, anti virus, nero etc, just your music software