Spring 2000, iirc. I'd been running a home NT4 mail / web server for about a year, and it was a royal pain in the ass. Half-life of about two weeks between bluescreens. Wednesday evenings dedicated to patches and defrags and reboots. Intermittent, unexplainable IIS freezes.
I was contemplating dropping a couple $K on new hardware, mostly out of desperation. At the same time, I'd played with Linux a few times, liked it, and it already had a well-established rep for stability. This was also the time the first commercial distros were coming into their own. I finally decided to take the plunge and bought (yes, actually paid for) a copy of SuSE, v.5 I think.
Steep learning curve; much swearing and regret; but when I finally put the beast online, it ran. For 14 months, and what finally killed it was a power failure too long for the UPS to handle.
In the nine years since (going from SuSE on a slot-A Athlon, to Mandrake/Mandriva on a dual Athlon XP, to nine Ubuntu VMs on a pair of triple-core Phenoms) I've had exactly two software-related crashes, one due to a misconfigured driver, the other from a runaway app that filled up /var. Uptime for this latest interation, which went online in Dec. is 100%.
And patch-the-server Wednesdays are a distant memory.