streaming movies skips/stops every time under Windows 7

Almost every time I watch iPlayer or a streaming movie site, the movie just stutters and keeps buffering, for about half a second. It repeates this thing every 10-30 seconds!

I tried the Microsoft drivers, and I have downloaded the newest Marvell drivers for my network card, but nothing changed. I have only IPv4 turned on on the card settings. It's a clean 64-bit install.

What;s the exact make and model of your video card?

It's a nVidia 9600 GT with the official Windows Update drivers (191.07). Using any movie player program, everything is fine, it only happens on streaming videos. Some of the sites even write "buffering" for half a second, then they go in.

Edit, I was not right with the VirtualBox XP part. It's stuttering there too, I was just lucky before not to have stuttering in VirtualBox. (I deleted that part from my post).

Well, the video card can't be the issue