New system extremely slow

Hello everybody,

two weeks ago I built my very first system after a rather long hiatus due to studies and other stuff. I did not actually build the parts together but started it from scratch out of the shop where I bought it.

The first days were really happy  

Now I start to wonder if I did something wrong. Everything went well but as soon as I tried to play a game (basically Battlefield 3) things became weird.

First off BF3 is absolutely not playable (extreme lag even in low settings when I have the chance to reach the game menu, sound echo and sound stutters). Starting it (be it online or offline) completely overwhelms my computer which becomes ultra slow.

And now when I use my computer for other things my processors seem to go very close to 100% usage for quite basic applications such as using Chrome, Word and let's say VLC together.

I was not expecting to play BF3 and launch CS5 at the same time but I thought my config was ok:
intel i5 2400 @ 3.1 Ghz
motherboard Gigabyte Z68XPUP-UD3 ATX DDR3
Kingston hyper-X 12800/1600 16 Gb
Sapphire HD6870 1Gb
SSD 60 Gb
HDD 1 Tb

I installed Win 7 on the SSD and most of the other applications on the classic disk. I'm using two monitors, one via DVI and the other via HDMI.

So I really don't know what's going on. Did i buy the wrong components to put together? (at least I could blame myself)
All drivers are up to date, the system is new so everything is "clean"..

I would really appreciate any help and thank you for your time!