>All my work is local (no network involved).
>
>Using Trend Micro Internet Security. (Am curious why this would be relevant and, if so, why it would just have started recently)
>
>>>I've been getting the following error message every day or so:
>>>
>>>Memo File (Form-Name).SCT is missing or invalid.
>>>
>>>This occurs after I have been working on a form, when I try to save it. It's not the same form every time or anything, so I can see no pattern.
>>>
>>>Now, obviously, the SCT had to exist when I opened the form to edit it, and I can see that it exists, so it must have become invalid.
>>>
>>>How can that happen, if I am saving it???
>>
>>What is the antivirus software you're using and are you saving locally or on the Network?

Usually the symptoms you indicate are related with either saving on a network or when these files are checked by the antivirus. Also make sure that you have read-write access to the place you're saving the forms. And it may help to do
compile form myForm.scx for all of your forms.