Today I started up my computer, but FOR SOME REASON my background changed to a huge crab over a town, with tons of little crabs on the image borders, with "The mighty crab of doom has awakened".
Now, I'm posting this here because the crabs on the borders are CORTEX COMMAND CRABS!
Virus?
Proof:
I want to add that the background changed while the computer WAS OFF. Or at least, when starting up/shutting down.
I didn't change it yet because I don't know where the file is located in.

I went to "change background" and has I expected, it showd the path to the image currently used.
Now, the path is "C:/crabsystem", which is a folder I never created with a picture that I never downloaded in there.
Guys, please check your computer and see if said folder exists in your disk.
I'm going to school now, see ya later (around 6pm, depends actually).
EDIT: Made a failed attempt at searching "The mighty crab of doom has awakened" on Google. I didn't get anything, not even in the images section...

Hahaha this situation is pretty hilarious, here's why:
Your problem, such as it is, came from here. This 'virus' is a great joke placed in the mission by its creator. When you kill the crab it simulates a blue screen, shuts down or locks (or maybe neither now) your computer and replaces the background with this one (you shouldn't have killed the crab).
You should probably have noticed a few other problems shortly before this (as mentioned a fake bluescreen, your computer locking/shutting down on you) but I guess you didn't realize they were connected. Anyway, you can change the background back and delete the crabsystem folder without any problems.

Well lol, It didn't change correctly, since It didn't do it after "blue screening".
I don't shut down my computer at all, I put it on sleep mode so it's faster to start using it.
Yesterday I had to do windows update, so I turned off the computer when going to sleep and only then it seemed to change the background.
Also, after putting the computer to sleep in the morning, it doesn't have the background again.
Anyways, forget this thread I guess.

Damn man this happened to me too and pissed me off... AND I DIDNT EVEN KILL ANY CRABS!!! XD

hey man dis happen to me to iw as rely pissed off to u no???? hahahahahhahaahahah

I acidentaly stepped on a crap and i got This but no bluescreen

HAHAHAHAHA lol i know this joke its from lab exploration hahaha gave me a shock to... damn scary eh? after awhile of research on the thing i thought it was hilarious and even decided to keep the crab system folder just for the picture! well anyway you must be using a old version of the mod when it was very unstable but its more stable now anyway have fun!