I've just started playing through SOA for the second time but it keeps crashing. The game screen disappears, leaving me back on my desktop with a message saying something like "runtime error - abnormal program termination". This never happened with my last game, but this time I've installed the concurrent romances patch from Baldurdash so I can romance Viconia while still having Aerie in my party, because I liked her in my last game (2 healers and 2 mages in three people are better than one of each I say). Could the patch be causing the game to crash? I hope not, because I don't want to kick Aerie out of my party.

That is not really a patch, but a tweak if you will. Did you install the Official Patch and Baldurdash Fixpack first before modding your game? Install them first, then reinstall your tweak.

Well it sounds like it and i'd say more so if your running windows XP which is known for its bugs and inconculsive crashes when using mods (well when using anything actually!).
Have u tried a reinstallation whilst preserving your saved games in a different folder?