There are two executables that trade control back and forth in TFTD: they're named "geoscape" and "tactical". There's a problem with "tactical" that causes crashes in any NT-based operating system; whenever a combatant falls any distance, whether it's visible to you or under "hidden movement", there's a very large chance the game will crash to desktop. It only happens under those circumstances, but... well, if you can't find the alien whose descent is crashing the game, you're basically boned. It happens a *lot* on my system, enough to make it unplayable.

For the record, that system is:

AMD Athlon 1.4GHz x 2 (dual proc)
nVidia geForce4 ti4400
Creative SBLive! Platinum 5.1
768MB RAM (DDR)
Windows XP (happens with any variation of Service Packs)

It works ok with the patches found on http://www.xcomufo.com/x2faq.html.

The crashes are normal. DOS version had them too -- bugs :)