Create a unique constraint on the two columns together, this will cause Access to reject duplicates as violation of the unique constraint. What you do with the error will be up to your form. There's a SO answer here:
The pertinent answer:
Open the table in design view in MS Access, select the three columns that you want to make into the unique index, and then click the little key on the toolbar. You cannot have null values in a primary key (set). http://stackoverflow.com/users/2548/remou
Edit since you already have a PK in place and probably don't want to create a new composite PK to replace it, but this is the other option:
Anyway, here's how you create a multi-column unique index on an MS access database.
1.Open the table in design mode, and Design, select Indexes.
2.Create a new row and enter a value in the Index Name cell,
3.Choose the first column from the drop down menu.
4.Add a new row and leave the Index Name cell blank.
5.Choose the second column, and so on.
Other (and bad) option is to query the database first and then not to insert, but that leaves the door open to potential referential integrity issues at a later date.
As a side note though; are you sure you want to insert for every combination of username(email)/password that is unique? This might cause problems for users that forget their password ending up with duplicate accounts. You might want to think of some other composite identifier (e.g. handle + password are unique AND email is unique to the DB. Or just simply have the e-mail address be unique. I don't know enough about your problem set - but it's something to think about.