Dude try this...
Boot your PC using Safemode (Press F8 on startup and select safemode)
Then login using Administrator account (default password is blank unless u changed it)
Go to Control Pannel-->User Accounts and create a new User Account with Administrator privileges.
Then delete your old user account... Unintall any virusguards that u installed before if you can..
Then restart and try log in using ur new account.. let me know what happen
If this able you to connect to internet Download Avast Antivirus and clean your PC.. Select Yes when Avast prompt you to scan your computer on restart at the end of installation..
http://www.avast.com/eng/download-avast-home.html