Missing Core in Task MAnager

[Clearcut]

Hi this is probably a common problem but I did a search and couldn't find anything.
I noticed yesterday that despite having a dual core processor - AMDx2 3800, I now only have one graph on the Performance tab. I updated the BIOS, checked ACPI is running, the dual core processor is showing in Device Manager etc but I can't seem to get the second graph back.
Has anyone got a cure for this please?

 

[Clearcut]

Thanks for your reply, just checked and it is listed as ACPI Multiprocessor.

 

[Clearcut]

I have just installed PC Wizard and got the following:

http://img.photobucket.com/albums/v334/groomys/untitled.jpg

Any idea how to switch the other core on please?

 

[Clearcut]

Yes I have, and the latest bios. This is really puzzling me. Would a clean install of XP solve this?

spec:
Athlon 64 x2 3800+
MSI MS-7252 board
2.00gb RAM
450w power supply (not sure about all the technical stuff)
BFG 8800GT 512mb

 

[Clearcut]

Update:
Just completed a fresh install of Vista Ultimate and there is still only 1 graph showing. Also, the CPU seems to run at 100% whenever I run an intense application like Winavi it reaches the 100% and crashes.

 

[Clearcut]

It is greyed out.
Also, with the new installation it now says ACPIx86-based pc. The miulticore reference seems to have disappeared.
I basically updated the bios and now I seem to have lost one of my cores that I cannot retrieve whatever I do.

 

[Clearcut]

Just an update.
Firstly It has shown two graphs in task manager for nearly 2 years so I am positive it's a dual core.
I have done a completely fresh install of, firstly Vista - no joy, then XP Pro. It was still recognised as a single core. I have cleared the CMOS - still one core, installed the latest BIOS, installed the processor hotfix/driver etc. Basically tried every suggestion I have been offered on the net.
I'm starting to resign myself to the fact that there may be something wrong with the processor.

 

[Clearcut]

Sorry guys, I am only offered 1 there is no 2
It seems that one of the cores has simply withered away and died. Could it be a dodgy hard drive? (clutching at straws here)

 

[Clearcut]

Hi, I don't believe this, I have just put in a brand new 4200x2 processor as I thought after trying everything, it would be a fault with the old one and despite recognising the new one in startup/BIOS it still only displays one in Task manager??? What on earth can this be? I have installed XP Pro twice, tried Vista Ultimate, updated the BIOS, bought new processor and still it won't utilise both cores.
If anyone can help I would be so grateful.