My HIS Iceq3 got delivered today. My system was due for a format and reinstall so I put the card in and just went ahead and foramtted. After XP installed I noticed everything lagging a little bit. So checked task manager and it had my 2nd core pegged at 100% at idle. None of the proceses were using it though. At first I was convinced it was a software issue, but then thought it couldn't be because there was only the bare XP install and drivers installed.  
 
So I did lots of fiddling in the bios changing various settings and still I had the same problem. So finally I put my old card back in and cpu usage was back down to 0% again. I have tried different PSU cables, changed the PCIE voltage etc and I still can't get it to work. I know its not a driver issue becuase it still happens with no drivers installed and only occurs with the 3870.
 
I'm usually on here helping other people but this one has me stumped. I have never heard or seen anything like it. I can't see how the gpu is causing the cpu to ramp up at idle. I'm sending the card back anyway and getting a replacement but I would still like to hear any ideas in case the replacement has the same issue.  
 
I can't test the card in any other systems at the moment. My guess is either the card is dodgy or my mobo doesn't like it.
 
Specs:
XP SP2
4800x2 @ stock
2gb OCZ platinum ddr400
Asus A8R-MVP
Enermax Infiniti 720W
 
Like I said if anyone has any ideas let me know. I'm not going to be around for the rest of the night to reply but I will be on tomorrow so cheers for any replies.

I think it would be useful if you could have identified the process that was using up all the CPU utilization. I have never heard or seen this kind of problem before, so hopefully the RMA will solve your problem.  
 
Cheers.

Thats the thing there is no process using the cpu utilisation. The system idle process shows 99%. 1% task manager. But the cpu usage shows 50%. And when I look at the graph its all on the second core. How can 50% cpu usage be used by on processes. I would post a pick but I havn't got the card in at the moment. I'll post a pic tomorrow cause its 1AM in the UK.

Update:
 
I have found that even if I disable 1 of the cores or set the affinity to a specific core the card causes that core to max out.

You have a Hardware Interrupt and DPC problem, my friend. I'm 99.9999% sure of it. To check download Sysinternal's Process Explorer. I will show Hardware Interupts and DPC's that Task Manager does not display.

I've never done this before, but if it's not a driver conflict, you may have to change the IRQ for the card. I'm not exactly sure how to do that... but I'll do some research.

Anyways, this problem is usually caused by two pieces of hardware conflicting either by drivers or interrupts, so if you change the interrupt to one that is not currently used by any other hardware then it SHOULD eliminate the problem.

*edit* I had another though: Go to the device manager while the 3870 is in and find it there. Right-click it and go to the 'Resources' tab. Look for IRQ #. Write it down, swap to your old card and then see what IRQ it is using. (You may have to do this with each instance of 3870 that appears in the Dev. Mngr.) **Also, check the little box bellow that says something about "Conflicting Devices" and see if it lists anything.**

Cheers I tought of that aswell but my knowledge of changeing IRQ's is limited. It was my thought exactly that there is a hardware conflict. However I think the problem might be all the way down to the mobo.
 
My reasoning for this is that:
I have Qfan enabled on my mobo. Everytime I boot it throws an error message about the fan. This is because my fan is running very slow because my temps are very low. So I just ignore it. However since installing the new card at post I no longer get the error message. So I went into the bios and the fan was running faster for some reason. So it is possible that from the second I boot core 2 is being loaded up causing the increase in temp to increase and therefore the fan to throttle up.
 
Thanks for the advice I will try it and hope it works.

I have just run Sysinternal's Process Explorer (which by the way I had never heard of but I like very much) and guess what hardware interupts is showing the core usage.
 
Next task is to find out how to deal with it.

I knew it. I had a similar issue with a mouse and got tons of "WTF, you can't have 50% CPU usage with no processes using that, etc. etc." and eventually stumbled upon the concept of hardware interrupts.  
 
Dealing with it is the difficult part, it's going to differ in each situation. Best of luck to you.

Well I've just put the old card back in and windows says that it has the same IRQ as the new card. But cpu usage is back to 0% again. If you have anymore ideas let me know.

Oh! I think I have an idea. The 3870's have audio built in right? Try seeing if there is an audio device that can be disabled. Either the on-board sound or the sound on the card (if you're is equipped). Maybe that is the issue?

I tried disabling the audio from the bios but that didn't solve it. However you bring uo a good point that I forgot about. When I checked my devices the card had not got its own audio. Normally when installing a 3000/2000 series card you have to switch the audio back to defualt because the card changes it. This is something that I have not had to do. The card doesn't appear in the device manager with any audio and neither does it appear in any of the audio settings.  
 
Edit: I forgot to say the card is assigned an IRQ 18. I thought there were only 15 IRQ's.

Did you try installing the drivers? Could be that the computer is having difficulty controlling the sound and video capabilities of your 3870.  You mentioned your drivers weren't installed... most of the time that does result in slower performance.  I'd try the drivers to see if they might be causing an isue.

Also, in your bios did you have Plug-n-play enabled or disabled. I would disable the plug n play operating system option. Let the BIOS assign the IRQs

Confirmed: asus mobo failure;  
try put your 3870 in another system. 99% sure that asus causes the problem;
 
P.S. Asus sucks!

I've done it. It was an issue with the sound. In the device manager there is a device in System devices called Microsoft UAA Bus Driver for High Definition Audio. I disabled it and bam the interupt is gone.  
 
Its crazy the amount of times I have told people about the audio on these cards and I forgot about it myself.

Awesome!
 

Yeah, I almost forgot about it my self, but then I remembered a few people complaining about laggy computers and their onboard sound no longer working.  
 
Glad you got it resolved though.

Its not a permenant fix though. For that I need the second version of the driver which can only be got through submitting to microsoft for the hotfix. So they already know its an issue. It appears it has happened to a lot of people.

 
 
lol  
 
no way! asus rocks!

thanks for digging up this issue. i'm also having this problem with the iceq3 3870.
 
i currently have onboard sound disabled and am using pci creative x-fi. my guess is that the x-fi is trying to hog the same irq as the 3870's audio device.
 
i will try to disabled plug'n'play from the bios, and see if it will do a better job of assigning irqs. otherwise, i'll move the x-fi to a different slot and see if i can give it a new irq that doesn't conflict.