So I'm pretty sure that you guys will have heard that the PC copy of MW3 was leaked a few days ago. I'm telling you now, DO NOT download this leak. Activision don't play around when it comes to pirating. They are tracing down the IP's of everyone who has downloaded the leak and will be fining anyone with possession. Here is a small segment from a website I found.

"He's hiding in your internets, snatching your games up, so Activision has decided to take it upon themsleves and are gonna be "knocking on doors around the country" and tracking IP's in the hope to give people the opportunity to hand over the game that apparently an employee working in a shipping warehouse in California was able to make off with the second disc of the 2-Disc game."

Apparently one guy from California has already gone through this and has been fined and stripped from online play. As tempting as it may seem, the game is out in just over a week.
Don't let temptation beat you.
Stay safe