Viruses, Spyware and the like can mess with the Windows IP Stack. the steps listed here are how to reset the IP stack in Windows Vista.
Go to the Start Menu, type cmd and right click or (Ctrl + Shift and hit Enter), and select “Run As Administrator”
Type the following commands, each followed by pressing enter.
ipconfig /flushdns nbtstat -R nbtstat -RR netsh int reset all netsh int ip reset resetlog.txt netsh winsock reset
Reboot the computer
Your system will get a new IP address and should be working as it was before!