There are a few options. I take it you have only got one drive connected at the moment. The biggest problem you have if that is the case is that both drives will appear to be c: You need to connect both drives but set the win 98 drive as master and the other as slave. When you start in win 98 it should show the old drive as "D". you will be able to access files from that drive via win98. If you want to use bo O.S. then you will need to create a dual boot and win98 help files will show you how to do that.
You need to be more specific about what you want to achieve as there are other options too.