I ran the distributed.net client for a couple of projects because I wanted to help them out. In July 1998 I was still running Windows 3.1 (true story!). Distributed.net didn't have a client for Windows 3.1, I still didn't have Win9x, and their DOS client, well, was unitasking, like DOS. I couldn't enjoy my PC and help them out too. I repartitioned hard drive space and started dual-booting with Caldera 1.2 (kernel 2.0.33).
Ran on a AMD K5-166, 96 or 128 MB RAM. Task switching was so much smoother than Windows (preemptive instead of cooperative multitasking, IIRC). I was hooked!
Today I spend 95% of my time in Linux (and 5% in XP).