Hello Zahra Mottaghi-Far,
The version you are using is before IE8 was officially released. IE8 was released in march 2009 and it is fully supported by RadControls from any later released version of the suite. This being said, you should upgrade at least to the first release after March 2009 or best - to the latest release so that you can take advantages of many new controls and features.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!