CHECK IF USER ALREADY LOGGED IN

below is an insert statement on an asp page that stores the date and time
that a driver logged on, what I need is to check that they are now already
logged on fields are SQL Server 2000

ID int
DRIVER_NO int
ON_DATE datetime
OFF_DATE datetime
ON_NOW nvarchar
SESSION_ID int

The ON_NOW column reads on or off depending whether the driver logged out or
not, if they havent we need to close the previous logon session and mark it
with 'off' and enter a date time into OFF_DATE column. Code: