Are you using your computers wired, and not wireless?
Also, have you tried just browsing the internet, without doing anything else on any of the other computers?
Finally, if it's a wireless router, make sure that you change the admin password and either disable wireless or set a password for it. Ideally also set the MAC addresses of your computers, as a further precaution. (I'm thinking that if wireless is enabled, then some neighbours may be using your connection, and downloaded stuff, slowing your connection.)