Ok I tried complete work for you and got suceed.
Take a look at complete query
create table TestDTbetween
(
autoid int,
start datetime,
enddate datetime
)
insert into TestDTbetween
select 1,'2011-01-20 10:05:00','2011-01-20 10:30:00'
union all
select 2,'2011-01-20 10:31:00','2011-01-20 10:55:00'
union all
select 3,'2011-01-20 10:56:00','2011-01-20 11:30:00'
select * from TestDTbetween where '2011-01-20 10:45:00' >= start AND '2011-01-20 10:45:00' <= enddate