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.
Oli