Hi and welcome to the forums.
First off, what make and model is your router; some routers (such as my secondary router) have a very low number of clients that are allowed to be connected at one time. It also may have some connection to how your router broadcasts (A, B, G or N) and how far away from it you are.
Please can you also tell us what OSs the other PCs are running and how they are connected.
As a general tip without knowing too much about your network, have you made sure that your WiFi card drivers are up to date? If you haven't it might be worth doing.