SC440 and video performance

Hi,

I purchased a Dell SC440 last year and could not use it for intented purpose. I am planning to setup HTPC (Home Theater Personal Computer) with this. Since this lacks PCIe 16 x, i purchased a PCI video card. The video looks good and i am able to use the DVI to HDMI cable and connect it to my TV. I also purchased a PCI sound card and it works too. I installed MCE 2005.

I am able to play the recorded videos such as WMV files in full screen model without any problem. No flicker at all. But I am having problem when I watch online videos such as www.cbs.com/hd or you tube online videos. The problem is happening in full screen mode not in the default window size. The colors are astonishing. But the video render is very slow. I see a very slow rendering. The CPU peaks to 99-100% when in full screen mode. I have very good internet connection. I also have two other laptops which plays the same videos fine in full screen mode. Is there any setting I can do in SC440 to make my full screen video perform better? The problem is happening both using my TV and my monitor.

The following are my SC440 specs:

925, 2x2MB Cache, 3.0GHz Pentium D, 800MHz Front Side Bus for PowerEdge SC440
3GB DDR2, 667MHz, 2X512MB 2X1GB Single Ranked DIMMs  - 2GB added by me
80GB 7.2K RPM Serial ATA 3Gbps3.5-in Cabled Hard Drive Primary
500GB 7.2K RPM Serial ATA 3Gbps3.5-in Cabled Hard Drive Additional  - added by me
Onboard SATA Controller
On board Network Adapter
48X CD-ROM Drive
DVD RW  - added by me

I performed the following steps to rule out some possibilities:

1) Removed video card and used the onboard video out to my monitor. It performs the same.
2) Removed the 2 GB RAM I installed and it performs the same.

Any help or suggestion with this regard is highly appreciated.

Thanks,

RT