When you try to reach the Win98 machine from the Win2K machine everything is fine?
But when you try to reach the Win2K machine from the Win98 machine it does now work?
Win98 by default does not support the authentication scheme in Win2K, but it tries with it's own username and password.
Therefore the username and password must match... I.e. on the Win98 machine set the username as "Administrator" without password if that is what you use on the Win2K machine...
Or, create a new user on the Win2K machine with the same name as the Win98 machine (Start>Run>compmgmt.msc>Local Users and Groups>Users.....)
Tip: always use a password for your computers, otherwise it becomes extremley easy to hack your computer from the internet, there is a share called $C with full access to all files on your computer....
(try to type \\compuername or IP-Address\c$ in the run prompt.....)