Not sure if anybody's revisiting this thread anymore, but especially for those who may have the same original question I did, here's the update. I followed BAD's and mikkele's advice about reinstalling, etc., and everything seems to work fine. Given my lack of computer savvy, I botched things several times, but after all is said and done...
(i) I got rid of all old MTW files on my computer (didn't have to do this, as it turns out, but "live and learn"), then
(ii) re-installed MTW, re-installed Viking Invasion, patched it up via 2.01 patch, then
(iii) simply did a copy of that entire folder 3 separate times, each time renaming it (once to "Hellenic Total War", once to "NTW" cuz I've also downloaded the VERY interesting Napoleonic mod, and once to "XL" -- the mod I play most often these days); I was interested to note that when I renamed a copied folder the computer didn't warn me that "renaming this folder might make the original program not work", then
(iv) whenever I want to play something other than the clean game I go into the particular folder itself and open the game manually by double-clicking on the black/white/red Medieval - Total War icon lurking amongst all the other game files... Took me a while to figure this out, I'm afraid, but got there all the same.
Anyway, thanks to everybody who helped out! Now if I could only get the HTW mod not to CTD whenever it goes to the battle map...