I was a PC only gamer until about 2 weeks ago when my brother got an xbox 360 (for sports games mainly) so I'm having just about the opposite problem. My suggestion is to just keep playing for a little while with the mouse and keyboard you'll get more proficient, don't worry

plus there are others out there who can aim to save there life (literally) so you won't be alone in your struggles.
My favorite things about PC gaming...
Mods, Custom Maps.
Dedicated servers. (great for clans)
Teamspeak/Ventrillo. (kind of goes with above)
mouse and keyboard.
customization.
Let me give you an example of why mods and dedicated servers are so awesome.
I've spent 25+ days of time in 'vanilla' COD4, but I spent over 50 days worth on one mod, specifically one clans server. In one week I had 78 hours on their server. . . I joined after that lol xD That server had a map rotation containing around 30 professional quality maps. several of the maps were made by a guy who now makes maps for treyarch.
The mod was awesome because it put all the weapons at military specs made you slower with bigger weapons and faster with smgs. . . shottys had decent range but lower power the farther away you were.
well this post was sufficiently long I think
