More computer problems...(windows login & domain name)

Okay, so I was a complete moron for doing this without knowing the consequences, but generally MS idiot-proofs most things in Windows. Apparently not enough for this idiot...

I brought my computer to my sister's house and was trying to make my computer part of their network. I found an option to add a new network, so I added a home network. I did it in a 2 or 3 step windows wizard...Seemed pretty easy, once I selected a personal network, all it asked me to do was restart to take effect.

So I restart, and get to my Windows login screen. I enter in my PW...MS responds with "User name, password or domain name is incorrect please try again." (paraphrased).

I have no idea how to get into my Windows now. I'm not sure how to rechange the domain to the old one (I assume that is the problem, but am definitely not sure). I tried to return to the previous configuration, but that did nothing for me.

Am I screwed?? There has to be a way for me to get back into my computer, right? Please help!!