This could be way off the mark, but have you forced your network connection to a specific IP address?
To check, right-click your network connection and select properties. You should have an item in the list box called 'Internet Protocol TCP/IP'. Select this item and click properties. The properties page should be set to 'Obtain IP address automatically' and 'Obtain DNS server address automatically'. (I apologise if you already know this kinda stuff... just trying to keep it simple
)