If you want immediate update on all clients right after data changes then you need some sort of notification system either in a polled or pushed manner.
You can implement push mechanism using for example WCF with callback contract. Your client PCs would need to implement relevant callback interface and be constantly connected to server PC's WCF service. Callback call could actually carry the new data. Each client needs to filter out notifications which resulted from that client's own changes. Push mechanism is quick and efficient way.
Check this stackoverflow answer for example of WCF callback.
Pull mechanism would require a background thread on all client applications checking the server for changes. You can use a separate database table with a version counter that would get incremented each time anything changes on the server. Client applications would poll that counter, compare with latest version they have and update the data when new version is discovered. It is much less effective mechanism though as you need to do the polling frequently and get all the data each time there is a new version. You can make versioning more sophisticated and detect what exactly changed but that can get complicated quickly with multiple clients. Overall it does not scale very well. It is generally simpler than push though and for simple applications with not too much data it would be enough.
