To find out what your current MTU is open an admistrative command prompt and then type
netsh interface ipv4 show subinterfaces
Then start pinging a site until you get the “Packet needs to be fragmented but DF set”, drop down until you no longer recieve this.
ping somesite.com -f -l 1464
Add the 28 byte overhead to the maximum value you used above, this gives you your new MTU size. In my case 1464 + 28 = 1492, the normal for a PPoE connection.
Then set this value in your router and also in Windows. To set the value in Windows 7 use:
netsh interface ipv4 set subinterface "Local Area Connection" mtu=1492 store=persistent
You should probably do some speed tests before and after just to see if that has improved.