Then you can read up on Samba in general at their website.
However, the specific part of the Samba manual that you want to look at is: Chapter 14: Managing Directories.
You can make the shared folder accessible by both Windows and Linux users, but they may be able to delete each other's files. However, if you set the umask for all files created in that directory to 0755 everyone will be able to access the shared files but not delete any of them but their own.
Also, on the topic of keeping your accounting applications running on native Windows 98 computers: I would highly recommend moving the Windows 98 sessions to virtual machines running on top of Linux through Win4lin. Win4Lin will run your accounting package nicely and provide you with a complete Linux footprint, insulating your hardware from Windows sessions and limiting damage from viruses and still giving you your critical Windows applications.
This was first published in March 2005