Enter key problem

 Hai,
    i working in a web application. My application has been working fine in vs 2005 . Now it is changed to vs 2008. In all my pages i have the following problem.
in my pages all the button click events are working fine. But when  i am using the enter key for that nothing is happening but control is going to the corresponding events. When i am using the F10  button i can traverse each line of code in that event but nothing is happening.
For example i have clear button for clearing the text boxes. When i click the button it is clearing the textboxes. But when i  use the enter button for that  control is going there but textboxes remains as such.
 
i don't know what i  can do.
 
help me...
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