If you are describing a password needed to log onto Windows, you would have to log on with an account that has administrative priviledges, so you can administer the existing accounts. If you do not have that account, Microsoft provides no formal support on how remove the password, for obvious security reasons.
There are third party tools that can be used to remove/change the password, but I'm not going into those details since I do not know if you are the legitimate owner of the computer.
If you need files off the hard drive in your computer you can take out the hard drive put it into another computer and there you have it! But if you want to go on your user profile without all the hassel this is a good programme you can put on a bootable CD: http://ophcrack.sourceforge.net this will not delete your password but tell you it... However i do not recommend using 3rd party software, try and remember what your password was previously before using 3rd party software.
I use something called "Kon boot" I have used it on Windows XP Professional and it works great!
This option however logs the user on by using a blank password but I'd think you'd be able to change the password once your in.
Or, you could always hit "F8" before Windows starts and select "Safe Mode" then choose the "Administrator" account. By default it has no password but it is disabled in Windows Vista and 7...
Which Windows version is it?
In Windows 98, simply log in a guest or something (any username should do) and delete the user's .PWL file. that file holds the passwords for that user, delete it and you can log in without a password!