Desktop Linux wrote up a list of 13 reasons of why Linux should be on your desktop. Here's a snip:
Cost -- Linux is free, and that includes all the apps. Microsoft is greedy. Vista Home Premium and Ultimate cost hundreds of dollars, even when upgrading from Windows XP. Moving up to Office 2007 involves handing over another bundle of dollars.
Resources -- Even the most lavishly equipped Linux distros demand no more resources than Windows XP. Vista is greedy: a single-user PC operating system that needs 2GB of RAM to run at acceptable speed, and 15GB of hard disk space, is grossly obese.
Performance -- Linux worked faster on my Dell Inspiron Core Duo than XP, at least the way XP worked out of the box. After cleaning out the bloatware and trading McAfee's Abrams Tank for the lightweight NOD32, XP and Linux (with Guarddog and Clam-AV) perform at similar speed.