You're looking to perform record locking. Once you've created the DB, go to Tools, Options, Advanced
You'll see a some checkboxes and a radio group on the right side (I'm using 2003, can't remember if it looks the same in 2000 and below, I haven't done serious Access development in a while.) You'll see the options for open mode, shared or exclusive (self expanatory) and also for record locking. What you're looking to do is "edited record" which will keep 2 people from changing the same row at the same time.
An excellent Access linky below (will save you $50 on the book):
http://www.katsueydesignworks.com/tu..._databases.htm
eidt: Also if you're using Microsoft's DFS (Distributed File System - replication software) it DOES NOT play nicely with access databases. You'll have to map directly to the share, instead of the domain share.