Hold in the Fn (function) button, usually in the lower left, and push the NumLk key (usually F11 on Acers, I believe). If you need more help give me a model number and I can be more specific.
You should be able to enter safe mode by tapping f8 while Windows loads, and be able to login from there. Once you're in, you can try to use system restore to take your computer back to a point prior to your user profile being corrupted OR you can create new user account to login to and copy...