this is a guide on how to start up call of duty modern warfare 3.
in three steps!!!! (faster as ever before)

1. start up your computer
doesnt matter if its windows xp or windows 7 or whatever you are playing on
(note: this does not work on Wii U)

2. click on your steam icon to startup steam.

3.if you are there there are several options.
choose the second one saying library!
if you have done this well there is a icon saying modern warfare 3
and another saying mp version of it.
click on it and enjoy :D

I hope this worked out well for everyone. and remember. Adolf Hitler stinks