This is what i see when i look at "your computers" for me:

GenuineIntel
Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz [Family 6 Model 26 Stepping 5]
(2 processors)

This is what i see when i look ay "my computers" for someone else:

GenuineIntel
Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz [x86 Family 6 Model 26 Stepping 5]
(8 processors)

This is what CPU-Z shows me:

Processor 1         ID = 0
   Number of cores      1 (max
   Number of threads   2 (max 16)
   Name         Intel Core i7 920
   Codename      Bloomfield
   Specification      Intel(R) Core(TM) i7 CPU         920  @ 2.67GHz
   Package (platform ID)   Socket 1366 LGA (0x1)
   CPUID         6.A.5
   Extended CPUID      6.1A
   Core Stepping      D0
   Technology      45 nm
   Core Speed      2668.8 MHz
   Multiplier x FSB   20.0 x 133.4 MHz
   Rated Bus speed      2401.9 MHz
   Stock frequency      2666 MHz
   Instructions sets   MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, EM64T, VT-x
   L1 Data cache      32 KBytes, 8-way set associative, 64-byte line size
   L1 Instruction cache   32 KBytes, 4-way set associative, 64-byte line size
   L2 cache      256 KBytes, 8-way set associative, 64-byte line size
   L3 cache      8 MBytes, 16-way set associative, 64-byte line size
   FID/VID Control      yes

It's as if 3 of the cores are dead or turned off. i am not aware of
changing any settings that might have caused this..

    Any suggestions will be welcomed as i am puzzled.

note: being puzzled does not make this a special day.

Its possible that some how you have restricted the cores used in either the bios or windows.  Reboot and enter the bios settings and see if one of the options in there is causing the problem.

If the bios is not the problem boot in to windows and press the start button and click help and support.

Type "Selective startup" into Windows Help.

From the results choose Run selective startup using system configuration.

Click to open system configuration.

Click the Boot tab.

Click advanced options.

The box number of processors should be unticked and then click ok.

Then click apply and ok.

Now reboot

Windows 7 Home Premium x64,  Intel core i7 920

12 gigs tri channel DDR3

Asus P6TD Deluxe Mobo

GTX 460 and GTS 250

i should have included this last night but was very tired.
i will try these suggestion after work this evening and
report back.

Iis there something more from cpu-z that
i should have included in my original post ?

Ahhh, the old banana-fingers problem.  I do that.