Forms and controls

Hi,
I am very new to the forms concept.
I have been working with calendar and worksheet.
I have been using this Now() function to update the current date in the calender
But if I want to restrict selecting previous date, then how should I do that?
Now I placed a button in the form which will override the restriction on the calender when a password is matched.
I also want to display the current date and time in the text box as I load the form.But I can't do that, when even i click on anything the form gets updated but not by default.How do I show current date and time on form load time?

Last, If I select to enable the calendar with the password, is it possible to copy the date and time in the sheet1 instantly?And if I reopen the form and enable password to enter previous date, it will again append the date and time below the previous entry?
Please guide me.
Thanks in advance.
Me.