Only one core detected (on my dual core) in CPU-Z/Core Temp/Prime95etc

So I just did a little overclocking at settings I'm fairly sure are stable (don't remember how long I ran prime95 then but I think several hours - about 6 hours today yields no errors), but I notice everything is saying I only have one core.

I never thought to check this or knew it was supposed to show multiple but I see in task manager -> performance, there is only one graph of CPU usage. I'm pretty sure it was this way since I got it so I never thought anything was wrong.
CPU-Z detects it as the correct processor - Core 2 Duo E8400, but on the bottom right shows 'cores - 1, thread - 1'.

Prime 95 defaults to bringing up 1 worker in a pretty recent version (25.9.4.0) which it's supposed to automatically detect multiple. Looking back, I remember multiple workers were opened before by default.

Is overclocking causing one of my processors to shut down/not register?
Default speed of the processor is 3.0 ghz, overclocked to 3.825 ghz currently.
Vcore is showing 1.320 V in CPU-Z (set to 1.35625 in BIOS).
All other voltages left at auto/default, RAM at rated 2.1 V.

Yeah that's odd. Should be 2 cores 4 threads.
Sounds like something is turned off, like what Twoboxer says.

If you disabled things in your CPU settings in the BIOS as well, I'd go check you know what was disabled (if thats the case).

I'm also assuming you're not using the factory heatsink, and that' you've tried lowering your clock speed and seeing if that helps?

I don't think I disabled anything that wasn't already but could be wrong. Would the factory default possibly have this disabled? I wouldn't think so since the board is definitely among the 'dual core era' - eVga 750i FTW. Has the default phoenix BIOS, whatever version, not sure.

I use a Xigmatek 'HDT' cooler, which some reviews pointed to as the best of the best. It was a bit of a challenge to try and get the coolant applied correctly but I think it did it decent. I've been running this CPU/MB since may 08, so I don't think it's any obvious hardware problem like that, seems likely a setting somehow.

Any help on what options control those settings would be appreciated.

I don't see how you could be using two cores when Prime95 and Task Manager think there's only one core.

1.4 prolly means "1 thru 4". I guess you hit enter, see what it says, and pick a new option if necessary.

Long shot but try "Start Menu/Run/msconfig"

Boot.ini tab, click "Advanced Options".

Nothing should be checked.

What motherboard and BIOS are you using?

Was this a completely new build? Or was the E8400 dropped into a previously single-core system?

Mobo is an eVGA 750i FTW, BIOS is whatever comes with that stock - phoenix something or other.

On a fresh install of XP, 2 cores were picked up instantly in task manager. However now I can't get internet to work for some reason on that new copy of XP.
So that definitely indicates to me it's just an error with windows' install process.

Unfortunately, he says he did that.