The leanest and fastest Linux Distro I've found is Puppy Linux.
The operating system AND apps all fit in 1gb, MAX. A minimal install gives Firefox and business type apps in under 512Mb With room to store your own files too.
Obviously, such lean code runs FAST!
Specifically the Puppy Bruno 2 has been able to run on desktops, and laptops alike. It even ran better (finding the USB Broadband modem) on the Eee PC than PuppEee which was designed for that platform.
It can be run from a CDROM, HD, or USB flash drive. My 8Gb Flash drive with Puppy Bruno 2 has Open Office, qbitorrent, complete 'rescue ware', and a full multimedia suite. almost all music, files, and video formats run natively.
The operating system AND my apps all fit in 1gb, MAX. (leaving 7Gb in a separate partition storage drive)
Such a lean, fast OS can revive even 486 PCs to "Windows" speed.
Oh, yeah; no more viruses means no speed degradation with use ;-)
The latest Puppy Lucid is Ubuntu-based if you're into compatibility with a major Distro.
I run Linux Mint i386 which is an Ubuntu version with all the codecs already installed and Open Office included as my main system.
Imagine loading the OS AND all your primary apps AND drivers in 20 minutes from ONE CDROM - Linux Mint!
New to the scene is Zoring OS, another Ubuntu based system with all the Compiz Fusion (heavy processor burden) eye candy and is LOADED with usability right off the single CDROM. Sweet!