Start cmd as Administrator. To check MTU issue:
netsh interface ipv4 show subinterfaces
This will give you a list of network adapters with MTU

To change the MTU issue following
netsh interface ipv4 set subinterface "YYY" mtu=xxxx store=persistent

Where “YYY” is a network interface name like “Wireless Network Connection”.

Sometimes you need to remove device driver manual, because Device Manager in Windows does not want to remove anything and gives you “Error 31”.
If this is your case, this is the guide for you. I’ve spent a sleepless night trying to figure out how to fix this problem..
In my case that was Network Adapter Drivers that did not want to work. Wireless adapter did have a yellow triangle and in properties it showed Error 31. All the attempts to remove the device or driver did not work – the device just appeared again. So I had to manually remove the drivers.

Run device manager find your driver file and *.inf file are used by this device.
55577595
55577595

For driver files you have to read providers. If provider is Microsoft you can’t touch driver and
driver service – you can only remove non-system drivers

Remove registry folder:
HKLM\System\CurrentControlSet\Services\

Next remove driver files (*.dll,*.sys) ,*.inf file relate to your device you can find in

C:\Windows\inf\<inf_name>.inf
C:\Windows\inf\
<inf_name>.pnf
C:\Windows\system32\drivers\<driver_name>.sys
C:\Windows\System32\DriverStore\FileRepository\
<driver_name>

Don’t forget to do full backup of registry and not completely delete driver files, but misplace them: put in C:\tmp

Information taken from this discussion: http://www.vistaheads.com/forums/microsoft-public-windows-vista-general/440447-error-31-about-6t04-wifi-adapter.html

Problem: Vista can not connect to wireless network.. the system can not see the DHCP server in router and getting 169.254.x.x ip address which is private and no use at all.
To resolve this go to registry, find
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{GUID}
Where GUID is a id for that network adapter – you will see the difference if another one is wired and connected.
Add new entry (DWORD 32), call it DhcpConnDisableBcastFlagToggle and assign value 1.
Change entry DhcpConnForceBroadcastFlag to 0.
Try to reconnect.