I share a internet connection with a roommate with 3 pcs using a linksys router. I had lots of problems sharing it . Every time he goes on line screwed up my connection.My internet get slow sometimes I couldn't even go on line I had to unplug the power cable from the route . I thought the problem the problem would be with my pc , so I decided to find out the problem and after some research I decided to install a zone alarm.Zone alarm blocks his pc to access mine and it got so much better, but I still having problem downloading from the web. My download starts really fast but slows down. Example: when I download a 10 mb file it starts about 350k/s but slows down after fews seconds and end up about 80 k/s. But when I plug the cable from the modem direct to my pc the download keeps the same speed .I really appreciated for any help or suggestions.

More than likely, the other two people on your router have some kind of file sharing program open like Kazaa or Morpheaus or iMesh or whatever, and instead of completely closing it when not in use, they probably have it minimized and are allowing other people to leech files from them which is slowing down everything.

I know that other 2 pcs download movies 24 hours a day but each one has its own ip. mine is 192.168.1.100 and the others 2 are 192.168.1.101 and 192.168.1.102.Is that the reason is slowing everything down?The router can connect up 254 pcs. If connect all together and turn them on at the same time what's gonna happen? Any other help or comment will be appreciated.

You only have one connection that is shared. If you connect all the available addresses it justs gets real real slow. Some connections just run better. I guess you could kill the other downloads buy shutting down the router occasionally. Unless you want to go to a router that can be configured to limit the amount of packets to each connection, there is no solution short of disconnecting the other computers.

Thanks for answers I am almost getting there. My other question is: If I purchase an extra ip from the cable co. and use it just for my pc will it solve the problem? Can we use the router to connect the other 2 pcs?

Probably not. You still only have one connection (wire).