Why does one computer on a router slow down the other 3 computers.?

I have ADSL2, the other computers are on wirelesss and mine is direct plug in.

Why does one computer on a router slow down the other 3 computers.?antivirus protection

Both above answers are right....

Adding my two cents: It like traffic on a highway, the more cars you have the slower it goes. Your router is like a small road with the computers you have. It won't take much to slow things down if one of the systems is hogging up bandwidth for bit torrents or gaming.

Why does one computer on a router slow down the other 3 computers.?computer repair

Anytime you have a LAN, ie. More than one computer connected to your Cable or DSL network bridge, it will slow down all of the systems when any one of the systems is in use on the Internet.

You are sharing bandwidth of your network bridge amongst all of the systems that are using the Net.

Doesn't matter if you are using OC192, the bandwidth will decrease when more than one is using the connection.
Probably means your computer is hogging the connection doing updates or sending spam. Do a virus and spyware scan to make sure it's not the latter.