New build teething problems

Hey guys.
I recently bought a whole new rig which consisted of:


Corsair 6GB (3x2GB) DDR3 1600
896MB XFX GTX275 XXX 670MHz
Intel Core i5 750 2.66G s1156
Asrock P55 EXTREME MoBo
1.5TB WD SATA2 WD15EADS 32MB
120GB SSUNG 2.5 SATA HM121HJ
Cmaster ATCS 840 Silver Case
GH22LS50 22X SATA BLK RETAIL

Building the computer was easy and everything went fine. However there seems to be a problem. Lets say I'm installing GTA IV, Microsoft offive, or extracting a large rar file, the whole computer seems to grind to a halt. My mouse lags, entering text lags and opening ANYTHING from iTunes to wordpad can take up to 10 minutes.
Its a mystery and really frustrating. I've updated all drivers, reformated 4 times, tried 2 different sata drives and still the problem remains. I've also removed the 3rd DDR3 stick and just left it in dual channel 4gig. Still get issues. Yet to clarify, booting isnt any longer than normal, when I managed to run GTA IV it runs fine. Does anything have any clues to this?
Its maddening since I spent so much on this and yet my old AMD system could handle this alot better.
Running windows 7 x64

But it never did this with my old system also running Win 7. I've got a better system and its running worse.

If we are talking RTM then thats the version I am using.
I've tried BIOS updates, driver updates, multiple reformats and still the problem persists. £850 on all this gear and I feel the system is slower

At the start I did. But I've tried 3 different drives and still the same problem.