Downgrade Windows 7 to XP?

Did you go and get the latest ATI drivers for your card? That can sometimes cause problems.

32-bit software under 64-bit windows is ran using a subsystem called WOW64. This isolates the 32-bit programs from the 64-bit ones. This prevents file system and registry problems. This is an x86 emulation layer, hence it will not run 32-bit applications as well as they would run under a 32-bit version of Windows. However, it shouldn't hurt the performance too bad if you have a pretty decent system.

I'm running on an AMD64 X2 4200+ and F.E.A.R. ran just fine under Windows 7 64-bit. I would definitely check into your video drivers first. Also, tests on the web have a lot of factors that can cause differences between what you see and what they see, including what the actual hardware was used in the tests, the manufacturer of the hardware (when it comes to video cards, ATI is the chipset being used, but who manufactured the actual card using the ATI chipset), and as stated, drivers.

In both cases I was using ATI drivers version 9.12 - forcing ATI HD4650 as there is no support for Mobility Radeons directly from ATI. To be honest I did not test it with the drivers released from HP - the manifacturer, as they were released in May 2009. I tested with 3D Mark 2001, 3D Mark 2003 and 3D Mark 2005 and used for reference notebookcheck benchmarks. But anyway these do not explain the difference in performance on the same system (mine) in both operating systems.  

The laptop is with the foolowing parameters:

Model: HP DV7 2220EQ
CPU: Intel Dual Core - P7450 - 2.13 MHz (1066FSB)
RAM: 2X2 DDR3 - 1066Mhz
Video: ATI Mobility Radeon HD4650 - 1GB GDDR3 - (Core:550/RAM:667MHz)
HDD: 500GB - Seagate 7200rpm

Usually, Windows 7 will perform slower than XP (whether it's 32 or 64-bit), but most of the time, it's not so bad that it degrades performance beyond playable. However, it depends on hardware and drivers.

The main test is, how well does the game play for you? Is it really bad to the point that you can see the difference in the FPS or is it just that you saw it go from 100+ FPS to 60 FPS? At that point, you shouldn't see a difference. Anything about 32 FPS is usually not discernible by the human eye.

Just curious as to if you actually noticed a drop in performance while playing as opposed to 3D Mark scores. I know that I've compared my 3d Mark scores to other peoples and I've never gotten anywhere near their scores, yet I know my games play just fine, so there are a lot of other factors (memory type, overclocking, etc). I know in your case, you're using a notebook. Have you compared your 3d Mark scores with the others to ensure they are using the exact same set up as you (notebook manufacturer, driver versions, etc)?

As a matter of fact I have compared the results, and it seems for a similar setup I get the same results in XP, as the results they claim to have reached in Vista or 7. That is what brought me to the conclusion that it is maybe the 32vs64 bits that makes the difference. So I intended to try it with a 32bit OEM Win7, but I am not sure that it will be legal to use my 64bit OEM Windows key.

Okay, so the tests you've compared against were running 32-bit Windows 7 whereas you are running 64-bit. That can definitely make a difference.

I guess the main thing is, is it such a difference that you can notice the degradation of performance in the application or not?

I believe you won't be able to use the 64-bit OEM key with the 32-bit as it's tied to the 64-bit OEM. It will probably come up as an invalid key. As it is, with OEM, you'll have to purchase a separate copy of the 32-bit version as OEM copies don't ship both types (32 and 64-bit) like the MS retail versions do.