Hi,
I posted in the Windows XP section a few days ago without much success, I suppose the CPU section is better suited for that anyways, pretty much I experienced a significant performance drop with 3DS Max on a Quad Core running on Windows XP SP2, whereas on VISTA the performance is normal... Below are the rendering times of a scene:
 
XP Pro 32 bits:
- E8400@3.2Gz: 1min32s - 100% usage on each core.
- Q9450@3.2 GHz with default CPU affinity (i.e. CPUs 0,1,2,3 are checked under windows' task manager): > 5 min... CPU usage is only 40%-50% on each core.
- Q9450@3.2GHz with affinity set to CPUs 0 and 1 only: 1min50s - better but still way slower than E8400
 
VISTA 32bits:
- Q9450@3.2 GHz with default CPU affinity (i.e. CPUs 0,1,2,3 are used): 1 min 10s - All cores are at 100%
 
My config: Asus P5B deluxe, 2GB ram
 
Other benchmarks (x264 encoding, Unreal Tournament) behave as expected (i.e. nearly 100% boost on x264, and slight performance boost with UT at CPU limited resolutions).  
 
On XP the load of the four cores is way too low leading to a drastic performance hit. Prior to testing under Vista I just thought  3DS max split tasks incorrectly resulting in threads busy waiting for each other rather than getting the job done, the results on Vista point to a performance issue with XP instead, on the other hand other benchmarks behave as expected.
 
Is there a known issue like this one with quads on XP and is there a patch available?
Thanks!

Is the SP2 install fresh ?
 
What other applications are running (processes) in the background ?
 
Use Process Explorer rather than the standard windows software to take a look.
 
Hope this helps.

You have installed the Intel chipset drivers correct?  Do you have the latest bios?
 
Vista probably comes with the Intel chipset drivers by default because of the newer os.  Xp on the otherhand may not.
 
here is the link to your boards drivers, http://downloadcenter.intel.com/De [...] l&lang=eng
 
Or here:  
 
http://support.asus.com/download/d [...] xe/WiFi-AP
 
Also make sure you have the latest sound card/video card drivers as well.  

First time I ever heard of this problem, VERY interesting. I get 4 cores 100% loaded on my QX9650 running Vista64 Business. Please post the problem as I am very curious. Sorry I couldn't be any real help.

Thanks for all the suggestions.
The installation of XP pro SP2 was not really fresh, i.e it had time to download most if not all the updates.
 
The BIOS is the latest which supports the Q9450, same goes for the intel drivers.
Nothing particulary fancy was running in the background: avast, steam, daemon tools, these were not running on vista though when I did the test.
 
I got the Q9450 back and will do some more testing, the intent was to compare the Q6600, Q9450 and E8400 running at the same speed and FSB, the 3DS max results are stunning... I'll post the results online soon.