Your target machines will have to upgrade to 2003 or you can purchase the VSTO - Visual Studio Tools for Office to get a license to distribute the Access Runtime to as many machines as you want. See
http://msdn.microsoft.com/office/understanding/vsto/
Typically, new versions of Access are backward compatible. Meaning if you were developing on 2000 and created an MDE, someone with 2003 could run it.
For a network, I hope you are planning to use Frontend / Backend, where your code is in the frontend and the data is in the backend. The frontend is stored on a user's machine. The backend is stored on a network share. This improves performance over having everyone open the same database.
You can secure all of this in a number of ways. Access has Security. People often complain about how easy it is to crack. But it can work. Our book, "Access 2003 VBA Programmers Reference" can help walk you through the set up. (I wrote the security chapter.)
Access claims some limits as far as performance. 50 concurrent users is where they draw the line. However, I have heard of claims of reasonable performance around 100, with the note that database is not generally hit by everyone at the same time.
Randall J Weers
Membership Vice President
Pacific NorthWest Access Developers Group
http://www.pnwadg.org