Who is your ISP? What is the brand and model of your modem?
Do you remember to power cycle (unplug for 30 seconds) the modem when switching the computer connected to it?
After a proper power cycle please show for one Windows computer ...
Start, Run, CMD, OK to open a command prompt:
(For Vista type CMD in the Search box after Start)
Type the following command:
IPCONFIG /ALL
[Note that there is no space between the slash and ALL.]
Right click in the command window and choose Select All, then hit Enter.
Paste the results in a message here.
If necessary use a text file and removable media to copy the results to a computer with internet access.