Has it been set to use a fixed IP address on your network? That might cause it to bump another user from their dynamically assigned IP address. Open the gateway's home page at 192.168.1.254 on the PC, and see what happens when you start the MacBook.
Why when i turn on my macbook air will it disconnect another users online game on a pc in the house?
Suggest that you reserve an IP address for the online game pc, so that DHCP will not assign that IP address to MacBook Air. In my home set-up, I reserve IP address for each and every frequently used computer and devices, and I start by assigning a nice round number (e.g. 100) for Network Attached Storage (NAS) WD drive, followed by the main computer of the house, and so on....
For T3200M router, the IP reservation page can be found in the advanced set-up page. You will need the MAC address of the PC to reserve IP address. The MAC address can be found in the network adapter property page.