here's what my system is: intel C2D E6600 processor
nforce 680i sli mobo (EVGA 122-CK-NF68)

patriot DDR2 2x1GB pc2-6400 800mhz
nvidia geforce 8800 GTS
Operating System : Microsoft Windows XP Home Edition 5.01.2600 Service Pack 2

pwr supply: OCZGXS700

ive tried alot to get both cores to run. ive tried reloading windows and forcing the HAL to both ACPI multiprocessor PC, and advanced configuration and power interface (ACPI) PC thats i tried both istalling it over the current version of windows and that did not work so i formatted my hard drive fresh installed XP so now in device manager it has advanced configuration and power interface (ACPI) PC under the the computer tree but still it only reads 1 core in task manager also in cpu-z in the selection box on the cpu tab processor #1 is gray and cores is 1 and threads is 1. and in my BIOS in the place where it shows the cores it only shows 1 enabled but i cant access the others to enable the second core. I have updated my BIOS but still nothing. please help.

CPU-Z 1.40.5 report file
Processor(s)
Number of processors 1
Number of cores 2 per processor
Number of threads 1 (max 2) per processor
Name Intel Core 2 Duo E6600
Code Name Conroe
Specification Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz
Package Socket 775 LGA
Family/Model/Stepping 6.F.6
Extended Family/Model 6.F
Core Stepping B2
Technology 65 nm
Core Speed 2399.9 MHz
Multiplier x Bus speed 9.0 x 266.7 MHz
Rated Bus speed 1066.6 MHz
Stock frequency 2400 MHz
Instruction sets MMX, SSE, SSE2, SSE3, SSSE3, EM64T
L1 Data cache 2 x 32 KBytes, 8-way set associative, 64-byte line size
L1 Instruction cache 2 x 32 KBytes, 8-way set associative, 64-byte line size
L2 cache 4096 KBytes, 16-way set associative, 64-byte line size
Chipset & Memory
Northbridge NVIDIA nForce 680i SLI SPP rev. A2
Southbridge NVIDIA 680i SLI MCP rev. A2
Graphic Interface PCI-Express
PCI-E Link Width x16
PCI-E Max Link Width x16
Memory Type DDR2
Memory Size 2048 MBytes
Memory Frequency 400.0 MHz (2:3)
CAS# Latency (tCL) 5.0 clocks
RAS# to CAS# (tRDC) 5 clocks
RAS# Precharge (tRP) 5 clocks
Cycle Time (tRAS) 16 clocks
Bank Cycle Time (tRC) 21 clocks
Command Rate 2T
System
System Manufacturer EVGA
System Name 122-CK-NF68
System S/N 1
Mainboard Vendor EVGA
Mainboard Model 122-CK-NF68
BIOS Vendor Phoenix Technologies, LTD
BIOS Version 6.00 PG
BIOS Date 07/11/2007
Memory SPD
Module 1 DDR2, PC2-6400 (400 MHz), 1024 MBytes, PDP Systems
Module 2 DDR2, PC2-6400 (400 MHz), 1024 MBytes, PDP Systems
Software
Windows Version Microsoft Windows XP Home Edition Service Pack 2 (Build 2600)
DirectX Version 9.0c

I believe XP home only recognizes one core, I believe you need to go to XP pro as each "core" counts as a physical processor.

the only thing is that it used to read both cores. but my drivers were damaged recently so i had to reinstall windows and ever since it only reads one core.

I have a machine with xp home and it's running a dual core. You must be missing drivers, either for your board, ( nvidia site ), or need to do a windows update. One or the other would be the first logical and easiest step in trying to cure the problem.

Nope Xp home will support multi cores, but only one physical processor, had to look it up.

Whilst it would be a pain, how about whipping the CPU out and testing it in another mobo, either a mates or a friendly 'pooter shop?

Right Click My Computer, then Click Manage, then in Computer Management, Select Device Manager, and on the right you should see the "Computer" Item - press the plus (+) next to it, if it says anything BUT ACPI Multiprocessor PC, double click on it and click the driver tab, then select Update Driver, (No if it asks to search online), then "Install from a list or specific location (Advanced)" then "Don't search. I will choose the driver to install" then on the following list select "ACPI Multiprocessor PC"

Best of luck mate

I think he already tried that and it didn't work. I'm having this exact same problem except I have a Pentium D and a ASRock motherboard. My friend also had this problem and someone else on this forum also. We have yet to find a solution so if you figure it out, please PLEASE post back and share. My friend got a new motherboard and said windows now recognizes both cores. I still have one core showing though...

Should be a simple BIOS thing that asks you to active both cores, or multi core support or w/e.

Have a look thru it all see if you see anything along those lines.

If that doesnt work try updating th BIOS to its newest version and doing the same.