cannot add Ajaxtoolkit.dll in VS2005

Question

Hi,
    To add Ajaxtoolkit in my toolbox, I perform following steps in toolbox

1. Add tag
2. Right click on this tag then   CHOOSE ITEMS
3. Browse the path of AJAXCONTROLTOOLKIT.dll

then I got following message


THERE WAS AN ERROR LOADING TYPES FROM ASSEMBLY   C:\AJAX.....
COULD NOT LOAD FILE OR ASSEMBLY "SYSTEM.WEB.EXTENSIONS VERSION......"

someone help me regarding this.

"I am using Vista home basic"