This is what i do when nothing works:
Code:
ipconfig /release
net stop "dhcp client"
net start "dhcp client"
ipconfig /renew
pause
right-click on the desktop, create a textfile, paste this in it,
save it as a .bat file and run it.
Name it whatever you like. I use "IP.bat" just for the simplicity of it.
It fixes most of my problems..pretty much the same as Ash suggested with a twist
Note: If you have a working connection you don't have to run it.