Wireless works on some of our macs but not one

We're having a problem at our school and I don't know all that much but am the tech person. So here it is:

3 of us use Macs. 2 of us have no problem with the wireless (one with MBPro one with a iBookG4) and one person's mac has an issue with the wireless dropping regularly. He's using an older iMac using Leopard 10.6.2 and he has to go and reset the router to make it work again. But, the other 2 computers are still connected when his is not.

Any ideas of what I should look for?

Just checked, I'm using leopard 10.5.2 and my wife has 10.5.8 if it makes any difference.

Welcome to the Forum. On the assumption that you have gone into the Network settings on each computer and compared the settings the next thing i would look at is the device that is providing the wireless network. Is it protected/Encrypted? And is there any other units connected in between the modem and the device providing the wireless?