What is the difference of Server.HtmlEncode and HttpUtility.HtmlEncode

There is no diffrence just Server.HtmlEncode calls  HttpUtility.HtmlEncode.
if you use reflector you realise that server object is kind of Httputility.

For more info:
http://msdn.microsoft.com/en-us/library/ms525347.aspx
http://msdn.microsoft.com/en-us/library/system.web.httputility.htmlencode.aspx