Driver installation error, Vostro 3750

I am using a Vostro 3750 laptop with (legal) Windows 7 x64 Professional.

After installing the OS and the drivers everything seemed to be working. BUT I have experienced huge problems with the speed of the computer. I have worked with other laptops with similar configurations and this Dell Vostro 3750 seemed to be very slow (An MS word opening shouldn't take 35-45 seconds, right?) At that time the Windows Experience Index gave 4,6 for the processor (i7-2630QM 2.00GHz)

But let's not compare it with other computers. I thought I made a mistake during the driver installation process. So, I re-installed the operating system and the drivers. It worked, the computer was super fast as it should be and the Windows Experience Index gave 7.x for the processor.

Unfortunately, a month later I had to re-install my system. I had to re-install my computer 3+ times "to be lucky" and to get a working system with normal speed. I did exactly the same thing every single time. Seemingly, this driver is the problematic one: http://www.dell.com/support/drivers/us/en/19/DriverDetails/Product/vostro-3750?driverId=PC7W1&osCode=W764&fileId=2967225788&languageCode=EN&categoryId=VI

There are 2 options: either the driver installation succeeds and I get a speedy system or it installs the driver and it seems to be working but it's insanely slow and the screen freezes for 0,5 seconds in every 5-15 sec. 

My computer is 2 years old now, whever I reinstall my system, I have to do it 3-5 times and then one of them will good.

To make things even more complicated my motherboard, CPU, RAM have been replaced due to hardver errors. (Total death of the machine). Even after the replacement I have this weird issue with the driver.

I tried to install diffrent kinds of windows 7 (x64, x84, pro, home editions, sp1). I tried to install it from USB, DVD. (All of them were legal copies from MSDN) 

The problem I'm experincing now, is that i have to reinstall my system and I did it ~8 times and I still haven't got my "lucky" one.

Could anyone give me some reasonable explonation what is going on? Because this current slow computer can not be used, even as a typewriter. (It's not able to follow my typing in MS word, and I'm not typing THAT fast :-) )