On the server, open up task manager to the performance tab, then on the client PCs open the dental software. Does CPU or memory rise a lot when that happens? If it's CPU, probably time to upgrade the server, if memory, maybe you can add more, but may also be time for a new server.
What about hard drive activity, is the hard drive on the server being hit hard when the software runs? If the hard drive is grinding away while the app opens or is being used, it may be worth upgrading to 10k or 15k RPM drives in RAID 5 or RAID 10.
Also you should check the Networking tab in task manager and see if it is using a high percentage of your network bandwidth when opening and using the dental application on the client PCs. If so, and you're on 100mbit switch it's probably worth upgrading to gigabit.