If the Timer control is within the UpdatePanel, and the buttons are outside of the UpdatePanel, then then you cannot disable the buttons within the Timer's Tick Event....
Well you can, but they will not appear as disabled when the response returns to the browser after the timer tick event has fired.
You should not be using Response.Write in your VB.NET or C# code. When you do, it is unlikely that the string supplied to the Response.Write method will be written in a valid place in the HTML page that is generated. (Usually the string is placed above the <html> tag or some other random place...which makes your html invalid)
I recommend using a Label, Localizable or even a Literal control to display the data instead. When you retrieve the data from Excel, set the Label.Text value to what you would have supplied to the Response.Write() method.
I'm not entirely sure why your buttons cannot be within the UpdatePanel?