My halfling Fighter/Thief has a regular longsword equipped, but when I try to remove it, it says I can't. I may have cheated it in, and I'll try a remove curse spell, but can anyone help me?

Try the remove curse first to see if that works. Sounds like a cursed item to me.

If everything else fails, use an editor to remove it forcibly:

http://www.sorcerers.net/Games/IWD/index_editors.php

I tried to use remove curse, but it didn't work. And it seems to have effed up my save files, as they are messed up in the editor. I've decided to go back to a save before I hacked it in, and everything seems to work okay now.