The class you used is for when you only want a piece of your code to be impersonated. If you always want to impersonate another user you don't even need to write code, there is a web.config setting for that.
Here is a link to a .net 2003 article (first one that came up in search) but it still applies:
http://msdn.microsoft.com/en-us/library/aa292118(v=vs.71).aspx[
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<identity impersonate="true">
userName="domain\user"
password="password" />
</identity>