spsite.cs

SPSiteDataQuery query = new SPSiteDataQuery();

query.ViewFields = "<FieldRef Name=\"Title\" /><FieldRef Name=\"DueDate\" />";
query.Query = @"<Where>
		 <And>
		   <Eq>
			<FieldRef Name='AssignedTo'/>
			<Value Type='User'>" +
			    SPContext.Current.Web.CurrentUser.Name
		      + @"</Value>
		   </Eq>
		   <Neq>
			<FieldRef Name='Status'/>
		 	<Value Type='Choice'>Completed</Value>
		   </Neq>
		 </And>		
		</Where>";
//Query only the Tasks List in each web.
query.Lists = "<Lists ServerTemplate=\"107\" MaxListLimit=\"0\"/>";
/*Specifying the MaxListsLimit as 0 means that there is no limit on how many lists
in the site collection will be queries. If you want you can limit this number to 
increase your performance.*/

query.Webs = "<Webs Scope=\"Recursive\" />";

//Specifying the row limit will limit the number of items which will be returned.
//query.RowLimit = 100;

DataTable results = SPContext.Current.Web.GetSiteData(query);