The password on login in Win98 is merely for networking. It is not meant to stop access to the computer.
I cannot remember how to stop it - something along the lines of changing the network settings (right click on Network Neighbourhood).
If you want to totally stop access to your computer then there is bound to be some third party software about. The BIOS password is quite secure but there are simple "back door" passwords to gain access.
I'm sure you will get a thorough step-by-step on removing the login box on Win98.
Hope this helps