I would like to create a calculation that would search a text field and bracket words containing ".htm" with the proper href tags. I wasn't getting very far.

I doubt that you will find any simple calculation that will work, especially if you can have more than one occurrence of .htm in the field. You will likely have to create a script that searches for the ".htm " text, and then counts back until it finds a space indicating the beginning of the reference name, and then insert the bracketting characters. Then, repeat until no more occurrences of .htm are found. And, of course, repeat for each record.