Cant watch videso online

Hello,
I can watch YouTube videos but unable to view the videos from some web sites.
The message I got is: "We're sorry, our content is not compatible with your computer configuration. To access this content, your computer must use Microsoft Windows 2000 or higher, with Microsoft Internet Explorer 6 or higher, or Mozilla Firefox 1.5 or higher.
My laptop is a recently purchased Dell 17" Inspiron running on Windows 7 with IE 8 and Mozilla FireFox 3.5.6 and protected by McAfee.
Any help would be greatly appreciated!