you can get some performance gains if you disable what you're not using in the BIOS "setup". Like in F2 Setup, when you boot up.
My 1.5Ghz Pentium 4 (512 RAM, XP Pro SP2) is NOT attached to the net, no printers connected.
Disable NIC, Disable onboard sound, Disable Parallel port.
Watch out for programs that load at Startup, especially WinAmp or Open Office, which can steal resources (RAM & CPU) and just about any Microsoft product, from Office, Windows Explorer, Internet Explorer ... all use major RAM, and does not always release.
Windows Explorer alternative with small footprint is Q-DIR, which even has 4 windows at once, and uses little RAM & CPU