My backup was casting a spell and I had her drink a potion at the same time, now she dosen't do anything, and this takes effect even if I reload a earlier game, get a different backup, start a new game, or reinstall the game, how do I fix this?

You aren't giving near enough information. By "backup" I assume you mean one of the joinable NPCs from the OC? Is it NWN 1 or 2? Is it only NPCs or pets/familiars? What exactly aren't they doing? If you've deleted everything and reinstalled, either you didn't delete correctly, or you're misunderstanding their behavior. It's hard for me to help when you give so little info.

Never winter Nights 1. A backup is a NPC that follows you around, I have the cleric, her name is Linu La'naral. Pets I don't know. I'm a thief and she's a cleric so neither of us uses a familiar. She Isn't doing anything during combat she doesn't cast spells, attacks, when I tell her to switch weapons she doesn't, When I tell her to attack she doesn't, when I tell her to heal me she doesn't. I re-installed the game but didn't delete any saved characters or games.