Dblookup and removing double booking

[RicCross]

Hello i hope some one help me out

im new to programming and access so please forgive me for being stuipid.

I made an access 2000 database for a booking system for a confrence room.
I have a table called tblbooking
in that table there is :
date, start_time, Finish_time

I have a form called f_booking that the user can add a new booking.
the form has the following boxes : txtdate,txtstart,txtfinish

after the user has added a new booking i would like it to check that there are no double bookings. I think this should go in the before update event of the form

I think I should be using a dblookup funition something like

If Not IsNull(DLookup(&quot;[Date]&quot;, &quot;booking&quot;, (&quot;[Date]=&quot; & Me!txtDate)AND( [start_time]=>txtstart AND [Finish_time] =< txtfinish )) Then
MsgBox &quot;Conflict&quot;
Else
MsgBox &quot; No conflict&quot;
End if

Pleases Help some one