I should also mention you never want to insert POST or GET data directly into your database. At a minimum you should wrap them in mysql_real_escape_string...
mysql_query("INSERT INTO $table (user_name, password, email)
VALUES ('" . mysql_real_escape_string($_POST["txtusername"]) . "', '" . mysql_real_escape_string($_POST["txtpassword"]) . "', '" . mysql_real_escape_string($_POST["txtemail"]) . "')")
But you should probably do some data validation first (like no spaces in passwords, valid chars in username, etc.) So check POST and GET for security and data validation.