Game runs slow on HTPC

Hello, I have a question that is baffling me, I am trying to run Warriors Orochi on my HTPC, but even at the lowest settings with 640x480 res it gets choppy and even skips whenever the light effects happen from hitting an enemy. My PC isn't exactly at the top of its game but it should get better performance than that:

Chaintech 9JCS Zenith
P4 2.8 Ghz
2 GB DDR400 RAM
ATI Radeon 9800 Pro
Hauppauge PVR-250

Now I would just shrug it off as my PC being too old, but what bothers me is that I initially ran it on my old Dell desktop and it ran fine even with the settings boosted up a bit. Dell Specs:

Dell Dimension 4400
P4 1.8 Ghz
1 GB DDR266 RAM
ATI Radeon 9800 Pro

Now at that time I had a Geforce FX 5900 in my HTPC (it was quieter than the 9800) which has roughly the same power as the 9800 Pro. However when I encountered the problems on my HTPC, I thought maybe the Nvidia drivers were at fault, and I had just replaced my old POS dell desktop for a shiny new custom built PC (another story). So I uninstalled the FX 5900, took my 9800 Pro from my Dell, popped it in my HTPC and installed the latest ATI drivers, hoping that the FX 5900 or Nvidia drivers were the weakest link, but still, same problem....
Could It perhaps be my PVR-250 TV tuner card causing some kind of conflict with the game or graphics drivers? I am not recording or running any kind of PVR app while playing the game. I am running all of the same programs in the background on my HTPC as I was on my desktop, with the only addition being powerstrip. Anyone have any guesses, its driving me nuts because the game looks great on the PC. Even on my old dell it looks sooo much better than on a console. But its a console port so I would much prefer to run it on my TV via my HTPC. Thanks!

 

Well I checked my CPU usage during game play its 40-75%. And its not throttling from heat either, its sitting at a fairly cool 48 degrees Celsius. Also I far exceed the minimum system requirements:

Pentium® 4 1.6GHz
256MB RAM
3D Accelerator with Hardware T&L and Pixel Shading supporting 64MB or more of VRAM, DirectX 9.0c+

And I even exceed the recommended:
Pentium 4 2.6GHz
512MB
3D Accelerator with Hardware T&L and Pixel Shading supporting 128MB or more of VRAM, DirectX 9.0c+

Any suggenstions on what to troubleshoot? anyone?