Password / Permissions On A Dropdown Box

Hi there, I have searched for this problem and whilst I can see several "round-about" ways to achieve what I want, none quite do it in a way I would like.

Essentially, I have a dropdown box, with a list of water types. What is selected from the dropdown determines the data from a lookup table that gets utilised in subsequent calculations.

The lookup table is protected and cannot be edited, however what I would like to do is for the last entry on the dropdown box, which could be called "special water" would then prompt for a password. If the correct password is entered than the last row of data corresponding to "special water" in the lookup, becomes unlocked and can be edited. If the password is not forthcoming the user cannot select special water and consequently cannot edit the corresponding row in the lookup.

I can think of a few long-winded ways to achieve the basic result, but nothing seems elegant....or fool proof

I had thought about a simple check box against the appropriate row in the lookup called "unlock" which would require a password to use. Once the password is entered, then it can remain open until the check box is clicked again whereby a password is required to "lock" it again

I should add that I use 2003.