Stimulus Pack is gone from MW2

Attach FMJ to the weapon you are trying to complete the challenge with. Then follow an enemy with your fire as they run for cover. Or otherwise try to get them behind cover once they are hiding. Often, you can easily spot someone behind a window or a wall when the barrel of their weapon is sticking out. Easiest to shoot through is sheet metal like the most walls on Scrapyard.

Modern Warfare 2 (in fact all the Infinity Ward / Treyarch COD games) are based on a heavily modernised variant of the Quake 3 engine.

This means that the server (and other clients connected to it) can send config data files to your machine and pass on weird settings.

There was an exploit some dude did on a hacked X360 version of the 1st Modern Warfare whereby he created a a "viral" config that had low gravity, extra weapon damage for certain weapons etc. that would pass from console to console.

AFAIK these transferred config settings are transient and will go when you close the game, but it is entirely possible that they could persist on the PC version.

Also, since this is all accomplisghed just by sending data about it's pretty unlikely (but not impossible) that a hacked server would use this to inject executable code into your PC.