If you are starting up your computer and it is not accepting your password, check to see if your Num-Lock is on. If your computer's keyboard reuses several of the keys for numbers, when the Num-lock is on you'll be typing the right password but won't see it since the password is usually hidden.
Dumbest thing ever but happened to me recently when I changed the keyboard on my desktop computer. On power up, the new keyboard goes into Num-Lock mode by default (doh!). It being a new keyboard, I did not notice that the Num-Lock LED was lit. Since it happened to be the Windows startup password there was nowhere to type and see what letters where coming out (if it was a web site log in, I would have immediately noticed the numbers instead of letters showing up in emails). What could have been a major disaster turned out to be beyond minor so check the Num-Lock LED before throwing yourself out the window.