Though I have never tried this, looks fairly reasonable:
PC World: How to Reset Your WindowsPassword
Reset Your Windows Password Using the Command Prompt
Boot your computer using a Windows installation disc.
Wait for the setup files to load, and select your language. Click Next.
Click the Repair your computer link on the Install Windows screen.
Select the operating system to repair, and note the drive letter on which the OS is installed (it is probably C: or D:). Click Next.
Click Command Prompt at the bottom of the list of recovery tools.
Now you need to overwrite the Sticky Keys executable with the Command
The instructions below use drive letter C: as an example. If your
operating system is installed on another drive, replace C: with the
appropriate drive letter.
In the Command Prompt window, type
copy c:\windows\system32\sethc.exe c:\ and press Enter.
copy /y c:\windows\system32\cmd.exe c:\windows\system32\sethc.exe and press Enter.
Exit the Command Prompt and reboot the computer. At the login screen, tap Shift five times. The Command Prompt will pop up.
net user username password, replacing username with your username and password with a new password.
Exit the Command Prompt, and log in using your new password.
To prevent another user from exploiting the same trick to reset your
password, you can restore Sticky Keys as follows:
Step through the instructions above to boot the PC using your Windows installation disc, open the recovery tools, and launch the
Type copy /y c:\sethc.exe c:\windows\system32\sethc.exe and press Enter.
Exit the Command Prompt and reboot the computer. Pressing Shift five times will now activate Sticky Keys instead of the Command
Prompt. Store your Windows installation disc in a secure location.
Caution: Resetting a password using the trick means you
may lose access to encrypted files, encrypted email messages, and
stored passwords for Internet Explorer and network resources. Remember
to back up your important data regularly.
Otherwise, there are countless utilities to reset the password from a linux disc or back up data from an old hard drive from a live disc (people above have mentioned some).
If you are certain that
1. Your username and domain are correct
2. Your password is correct (and you don't have something like capslock or numlock on)
3. That no one (including yourself) has changed your password
Then you have a bigger issue that may be related to a corruption of important system files. That could be hardware issues, software, viruses, etc... You may want to look into that. Oh, and start making regular backups.