Just to point out for others, Chrysalis, as already pointed out, has done the right value specifically for the OP only. The value is only correct if you are using IPv4, not IPv6 - but this is WinXp, so very unlikely to be using IPv6, and it's only correct if you are using this particular MTU for PPPoE. Anyone else using WinXp then please shout, as you may or may not want a slightly different number. This is because figure is a multiple of x where x depends on your circumstances.
But modern operating systems do the right thing now anyway so there is nothing to think about for post Win-Xp users.