Also, I'd suggest checking out the Option Compare statement in VBA Help.
When set at module level, it will determine the default comparison method to
use for comparing strings so that you do not have to set/reset this option.
Just to make sure that I have interpreted your question correctly ...
In: InStr(1, txtCol, "maint", vbTextCompare) "maint" and "Maint" are
considered equal. So, if the value of your control, txtCol, is: 123Maint,
the expression above will return 4.
In: InStr(1, txtCol, "maint", vbBinaryCompare) "maint" and "Maint" are
not equal. if the value of your control, txtCol, is: 123Maint, the
expression above will return 0.