^ ^ ^ This would be helpful.
First thing I'd try (because it's quick, and won't break anything if it doesn't fix this) is resetting the network stack.
Open a command prompt (for Vista and Win7, an Elevated command prompt), and run the folllowing command:
netsh int ip reset C:\resetlog.txt
Give it about 5 minutes to finish, and reboot.
If it's still broken after that: please post the output of ipconfig /all from a command prompt - post both the broken machine, as well as one working machine (please indicate which one's which.)
To do this:
open a command prompt (Start > Run > [b/cmd[/b] (or, <Winkey>+<R>)
Type ipconfig /all <enter>
Right-click anywhere in the command window > Select all > <enter>. The window's contents are now copied to the clipboard; simply paste into reply.
(for the broken machine, you'll want to paste that into, say, notepad, and sneakernet it on a flash drive to a working machine.)