Change settings without access to user account?

I was playing around with my video drivers last night trying to get a game to work in wine, but my question is not about that. When I booted up tonight (I'm set-up to auto log-in) I get some strange video behaviour which makes my sytem unusable (a bunch of glitched out "water colors" instead of a desktop). I manged to get logged out via some key smashing. When I get back to the log-in screen my video comes back to normal. From here I can log in to the guest account, which uses the default x.org driver (I have my user account set to a proprietary Nvidia driver [version 319-updates]). I'm confident that if I can change my user account back to a different driver (version 319 worked well before) that all will be well, but I can't figure out how to do that from outside of the user account. I'm assuming that I can run some sort of terminal commands from my guest account to do this, but I'm too new to Linux to even know where to start.
Thank you!