My 2 Computers can not Access Internet Simult

Hi I have two (2) computers (Vista and XP); but can not access Internet Simultaneously. The 2 computers are connected to a Linksys Network Hub which is connected to the IO Modem. After I have changed carrier, only one computer works at a time and if I need to the internet to update the other computer, I have to configure or reset and turn off the other computer. I am just new to Networking and I will be glad and thankful if you can help me find solutions to my problem.