The DNS you use might be screwed up, if you're using your ISP's it's even more likely.
You can change DNS servers on a per-PC basis by editing the connection properties.
http://kb.epson-europe.com/pf/12/webfiles/Article%20Images/Networking/Windows/Win%20Vista%20screens/Ad%20hoc/wireless_connection_properties-Vista.png
go on "Internet Protocol(tcp/ip)" settings, (for IPv4 if it shows more than one).
at the bottom where it says "obtain DNS automatically" set it to "use the following...."
put: 8.8.8.8 on the first one, 8.8.4.4 on the second one.
open the command prompt (cmd in start menu) and do "ipconfig /flushdns" without the quotes. If you still have the problem then it's unlikely to be your computer. (Do a restart to be 100% sure it took effect though).
To undo the change just set it to automatic again. I suggest keeping those dns servers though. It's googles and they work awesome.