You will probably not be able to access the Vista PC from Windows 98SE, but should be able to work the other way round.
In 98SE, right click "Network Neighborhood" and select Properties. You will see a button marked File & Printer Sharing. Click that and ensure you have one or both boxes ticked, as appropriate. Presumably you have "shared" any folders you want to have available over the network? It is generally recommended not to share an entire drive.
Any firewall software needs to be configured to allow access to the networked computers, which may involve adding their IP addresses in a "trusted" section.