If you wish to generate a report which focuses in on Windows 98 systems running on your system then simply:
Access the Operating Systems :: by IP report
which can be found at Network Inventory -> Advanced reports -> Operating Systems :: by IP
To focus on Windows 98 systems then set the Description field to ^98 and then run the report. The report will run and list any hosts found which are running Windows 98. If this report is useful then select the Save Report from the More Actions menu, assign this new report a name and save it. Your new Windows 98 detection report will then be found under Custom reports.