Try this
DECLARE @timedata as table (mydate datetime , mytime datetime , myvalues bigint)
DECLARE @timedata_Formated as table (mydate datetime , mytime NVARCHAR(255) , myvalues bigint)
insert into @timedata select '2012.01.12','10:51:33.000',100
insert into @timedata select '2012.01.12','10:51:45.000',200
insert into @timedata select '2012.01.12','10:52:28.000',300
insert into @timedata select '2012.01.12','10:52:41.000',200
insert into @timedata select '2012.01.12','10:52:58.000',100
INSERT INTO @timedata_Formated
SELECT mydate,CAST(DATEPART(hh,mytime) AS NVARCHAR(50)) + ':' + CAST(DATEPART(mi,mytime) AS NVARCHAR(50)) + ':00',myvalues FROM @timedata
SELECT mydate,mytime,AVG(myvalues) FROM @timedata_Formated Group by mydate,mytime