I have a file on NTFS that holds accumulated information that will take forever to recreate. I marked it as 'archive' and 'system', so that it would not be obvious to notice or mishandle, but what I really want is to put some kind of lock on it, so that deleting it would only be possible when providing a password or fulfilling some other condition.
Is there a tool that can help me? perhaps some hidden Windows feature?
Just remove modify permissions from anyone but the user account that creates the file. This kind of thing is exactly what NTFS rights are for.
"accumulated information that will take forever to recreate" - the time to prepare a backup / restore procedure is right now. Don't wait until the file is somehow deleted or overwritten, or the disk fails. A proper backup strategy with proven (and tested) restore capability is a must.
Under NTFS you can alter the access rights for users and groups either at a folder level or at a file level.
Right click on a file or folder and go to the security tab. You can then remove users rights for deleting / modifying a file. Ensure you leave the administrator and administrators full rights. Also, if it on a domain ensure that both the local administrator and domain administrators have full access rights.
If you want a full logical protection, since Deny takes precedence on Allow, File Properties > Security tab > Advanced Security Setting , Deny to Everybody the delete right on this file.
Again, a full backup of this file is in order.