I'm not reading correctly again
If you have a dual boot, grub should already have been there allowing you to choose between Ubuntu and Windows.
Are you using an encrypted home directory? Are your login problems related to the graphical environment only or also to the consoles?
In case you don't know
At the graphical login screen, press <ctrl><alt><Fx> (x being a number from 1 to 6). You will get a text based screen stating 'login:'. Type your username and press <enter>. Next type your password and press <enter>; note that, while typing your password, you will not see anything happening; just type it.
You can usually go back to the graphical environment with the key combination <alt><F7> (sometimes it seems to be <alt><F8>).