No. ABVFP is a FORM PROCESSOR. A login script consist of various scripts, all working together, with database storage of the info, and MANY checks (unique username, password, email, encryted email, forfot password feature, cookies, sessions, and so on.
ABVFP can process the info from a form, meaning send you an email and store the info in the database, but it cannot be used as a login script.
If you are interested, have a look at
http://www.dbtechnosystems.com/my-solicitor/menu1.php which is such a login i created.