Alaster,

If you want to do this internal to VFP, try this...
Create a login form that sets a public variable llogin to .t. if the user logged in correctly.
Be sure to put a clear events in the closure of the form

In your entry program, set up all your defaults and values.
Structure a program as follows:

llogin = .f.
Do Form loginform name ologinform linked
read events
release ologinform
If llogin
Do Form main name oMainform linked
read events
release oMainform
EndIf

***********
The only way llogin is .t. is through the loginform
And if they don't login correctly, they never open the main form.

Hope This helps,

Ron