Hi Jane,
I'm not sure where you are with this, so we will try it step by step. And kiddy language suits me as I am devolving back to a child very fast, 
Do you know your emal settings from Netscape's Internet Service?
Is it a web mail service, (read and send emails from a web page), or do you use another email program similar to but not the same as Outlook Express?
If its a web mail service I may have problems helping you because I don't use them myself. But if it is a stand alone email program on your computer like OE, then the POP and SMTP settings should be the same for each program.
When you want to reply to an email are you using Internet Explorer at the time? If so, there may be an easier way to solve this. In Internet Explorer, goto Tools > Internet Options. Click the "Programs" tab and under "E-mail" find if Outlook Express has been selected. if so, select Netscape Mail from the list of options. This may solve your problem at a stroke, but the only way to know is to try it.
If not, then we need the POP and SMTP settings from your usual email program. If you cannot find them from it, (look under Tools > Accounts, or similar), then perhaps Netscape's Home Page will have them for you.
When you get them, open up Outlook Expres and goto Tools > Accounts. Click the "Mail" tab, and highlight your email account then click Properties. (if there is no account, see below).
Under the Servers tab, type in POP3 under "My incoming mail server is a xxxx server". If Netscape say that your incoming server is HTTP, then type in HTTP there.
For Incoming mail, (POP3), type in your POP server name
For Outgoing mail, (SMTP), type in your SMTP server name, (they may be the same).
Under "Incoming Mail Server" type in your normal username for the email account you use. Then type in the password underneath. Tick the "Remember password" option.
That should be sufficient. Click Apply, then OK. Close down OE and open it up again to re-try it.
If your OE has no email account, click the Add button, then Mail. The Internet Connection wizard will open. Follow the instructions and complete the details as above.
This sounds complicated, and definitely not kiddies language, but if you take it step by step hopefully it should help.
Let me know how you get on.
Mark