Here's another sample function you could use (and create something similar for dates and times) to make your life easier when putting SQL strings together...
Function EnquoteString(strText As String) As String
Enquote = "'" & strText & "'"
End Function
Which would be used like so...
Sub MyComboBoxQueryRowSourceThingee()
Dim strFirstName As String
Dim datBirthDate As Date
Dim intFavoriteInteger As Integer
Dim strSQL As String
strFirstName = "Marco"
datBirthDate = #6/28/2005# 'Note this is American format: Month/Day/Year
intFavoriteInteger = 19
strSQL = "SELECT peeps.strFirstName, peeps.strLastName, peeps.datBirthDate," _
& " peeps.intFavoriteInteger FROM tblPeople AS peeps" _
& " WHERE peeps.strFirstName=EnquoteString(strFirstName) _
& " AND peeps.datBirthDate=#" & datBirthDate & "#" _
& " AND peeps.intFavoriteInteger=" & intFavoriteInteger & ";"
Debug.Print strSQL
End Sub
I find using functions like this improves readability, and more importantly, it reduces mistakes.