We will need a bit more info:
a) Are you referring to logging into a password protected website?
b) Logging on to the computer as a user other than admin?
If it is A, deleting the cookies should stop this from happening. Often windows asks you if you want to use the auto-logon function when you first type in the name and password. I forget what the file is called but all passwords are stored in one file on your hard drive. I think windows help will tell you what the file is called and where it is. Delete this and your password is gone.
B One way is to remove yourself as a user on the computer. Unless you are using it all the time, then you will have to have the admin modify your profile to stop windows from (once again) auto-logon(ing) you in. I believe this can be done from the network admin area of the control panel.