ok, restart your computer like you would normally (just restart). when you get to your desktop, don't do anything except check activity monitor, making sure no other apps are open. Wait there for a little while to see if windowserver starts using the CPU a lot. if your CPU usage is still 40-70 (which is abnormal), then force quit windowserver.
Based on my MBP, which has similar specs to yours, windowserver is using 1.6%-3% CPU. This is with Safari (5 tabs), preview, PSCS3, mail and activity monitor open.
Also, if windowserver DOES end up using 40-70% CPU after your restart, check to see if you have any programs running in the background that might be crashing without you knowing. Like a virus scanner or something. I once had similar problems and it was always with my virus scanner running in the background hanging up and the system wouldn't tell me.
Also downloading Istat Menus is helpful I find, in monitoring your CPU usage and temps:
http://www.islayer.com/index.php?op=item&id=28
Whenever your comp gets sluggish, it's because of two reasons, one, your cpu is being used way too much by something, and two, your ram is very low. For you it is definitely the CPU.