Basic Datetime Comparison

Hi, I´m new to SQL Server, and I´m having the following problem. I have this query:

SELECT
P.DIMENSION, P.CUSTACCOUNT, C.NAME AS CUSTOMERNAME, PE.PROJID, P.NAME AS PROJNAME, PE.EMPLID, E.NAME AS EMPLNAME,
PE.FECHAASIGNACION, PE.FECHADESASIGNACION
FROM
PROJVALEMPLPROJSETUP PE, EMPLTABLE E, PROJTABLE P, CUSTTABLE C
WHERE
PE.PROJID = P.PROJID AND PE.EMPLID = E.EMPLID AND P.CUSTACCOUNT = C.ACCOUNTNUM AND ((PE.FECHAASIGNACION >= '01/01/2006' AND PE.FECHAASIGNACION < '19/09/07')
AND PE.FECHADESASIGNACION >= '01/01/2007')
ORDER BY
P.DIMENSION, P.CUSTACCOUNT, PE.PROJID, PE.EMPLID


When I execute it, I get this error, regarding the date comparison I´m trying:

The conversion of a char data type to a datetime data type resulted in an out-of-range datetime value.

How do I compare a string value to a datetime from the database? Thanks a lot.