Router/Network Problems

Ok Comp A(this one) and Comp B are connected through the same router. Yet Comp B cannot go online.

Comp B HAS been able to go online before and was fine for over a month or so. My sister uses Comp B and she installed some pop up blocker which i removed. But ever since she restarted her computer, it has "Limited, or no connectivity". I have no idea why this happens and i restarted teh computer many times. From what she gave me, she didnt seem to do anything wrong.

Comp B is online with "Limited or no connectivity". Meaning she is online but cannot go anywhere. Firewall i am pretty positive is turned off and i havent touched the router besides moving the cable around a bit. Router light seems to indicate that Comp B is there but not blinking as when information is being transferred.

Similiar experiences: I had this before on Comp A, but that was cuz the firewall cannot be turned off in which a Dell Tech told me to reformat and now works fine. Comp B is older and has alot of files i could need >< So reformat is the last ditch effort...

Any ideas?

On comp B, do the following: open a command prompt (startup menu > run > cmd) and type "ipconfig /all", then check what is in gateway, DHCP server and DNS servers, do the same on comp A, then post the differences.

They are different, i know that from someone else, its the router not reaching comp B and i tried doing iprenew and dont work, so i tried putting cable B in modem no dice..so i am trying fresh install since someone sugguested it was a windows prob..>_> fresh install how to do?

What do you get when you do ipconfig /all in comp B?