Navigate the the location below in your registry and make the corresponding change.

Create or edit the existing DWORD value (string value for Windows 98) named "PMTUDiscovery" and set it to equal "1" to enable discovery or "0" to disable it. 

Restart Windows. 

Key:[HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VxD\MSTCP]
Value Name: PMTUDiscovery
Data Type: REG_DWORD
Data:(0 = disabled, 1 = enabled)