Pass criteria from a value on text box in access form to a query to pull exact match result or All when left blank

Hi There,
I'm trying to pass a criteria in an MS access query from a text box in form. What I like to do is check exactly for the value user enter in the text box( i don't want similar value of what user enters, say if the user enters ABC, I want result eactly matching ABC, not like ABC such as ABC , ABCD, ABCDE etc, if they leave it blank( if needed I can use something like "ALL")  then pull everything.
On the query I have it like this    [FORMS]![usrpartform]![usrPart] OR Like "*"   but  it always pulls everything.
I would appreciate if you could tell me the exact syntec to be used on the query to get the result as desired.
Thks