Definitions of w00t on the Web:
* The term "w00t" is a slang interjection used to express happiness or excitement, usually over the Internet. The expression is most popular on USENET posts, multiplayer computer games (especially first-person shooters), IRC chats, and instant messages, though use on the World Wide Web in the form of weblogs or in forums is by no means uncommon.
en.wikipedia.org/wiki/W00t
I don't know if this is the origin...
But back in the very old days (like the late 70s or 80s) there was a series of role-playing games collectively known as "The Fantasy Trip" from a company called Meta-gaming. Meta-gaming producted Car Wars (before Steve Jackson Games) and that big tank game "Bolo"(?). This was back when Steve Jackson and Howard Thompson were partners or at least worked together. Their games were regularly advertised in the sci-fi periodical "Analog".
Anyway, one of the sentient/sub-sentient races in The Fantasy Trip (includes "Wizard", "Advanced Wizard", "Melee", "Advanced Melee" and "In the Labyrinth") was the Prootwaddle. They were described as something that looked about like a Tele-tubby (before tele-tubbies) without the head gear and with brown or natural colored fur. They had an IQ of 6 where 8 was the low-end of human-normal and would follow others around happily, joyously singing out, "Woot, woot, woot, woot, woot, woot." Somewhat like a deeper pitched guinea pig.
Meta-gaming broke up and Steve Jackson started Steve Jackson Games. "The Fantasy Trip" had copyright issues, so GURPS was created, but SJ apparently had the copyrights to Car Wars. The very active Steve Jackson Games BBS eventually grew into Illuminati Online (io.com), a major Texan ISP. And the SJ BBS was raided by the FBI in a notorious case which ultimately involved the EFF in one of its first court cases, because the FBI thought a role-playing book in the cyberpunk milieu was a cookbook for computer hacking.
The point of the last paragraph being that there was enough notoriety and connectivity from that source, for the few folks who know about prootwaddles to have spread the "Woot, woot, woot" meme into the general on-line community.
So, it may not be the source, but that's my 50 cents on the topic and my hypothesis of woot's source.