Could you post query execution plan for both cases?

<snip>

>Here is the command that is affected by the index when it becomes non optimized:
>
>

>DECLARE @NoMember Integer
>
>SET @NoMember=1
>
>SELECT Thread.Numero,Thread.Title,Thread.Url,Thread.AddDate
> FROM (SELECT TOP 10 Reply.NoThread FROM Reply (NOLOCK)
> WHERE Reply.NoMember=@NoMember ORDER BY 1 DESC) Temp
> INNER JOIN Thread ON Temp.NoThread=Thread.Numero 
>

>
>Right now, the result is:
>
>Duration: 1.7187 Count: 10
>
>When I index again, for a few hours after that, the result would be optimized such as:
>
>Duration: 0.0156 Count: 10
>
>The index that is affected is as follow and is at the end of the script just before the last index that is being built with the INCLUDE syntax:
>
>

>CREATE INDEX [IndexCombo1] ON [Reply] ([NoMember]) INCLUDE ([NoThread]) ON [Reply]
>