Backup Scheduler 98 - description from Softdepia.com:
Overcoming some limitations of Microsoft Windows 98's excellent Backup and Scheduled Tasks utilities, such as the inability to schedule an unattended backup job, and to backup files that are in use, this applet schedules and starts an unattended backup job any time you would like. You may choose to run differential backups nightly and full backups once a week. You can even setup the reminder feature to urge you to take your backup media off-site. This product will help you to realize the full potential of the Microsoft Windows 98 Backup Application.
Backup Scheduler 98 is a "Free to try" program. You can download and try it for an evaluation period.