NT sets the MTU to 1500 bytes which is appropriate NIC cards but not for modems. Cut and paste the following code into mtu.reg file and run it.

REGEDIT4
BLANK LINE GOES HERE
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NdisWan\Parameters]
"IPMTU"=dword:00000240
"TunnelMTU"=dword:00000240
BLANK LINE GOES HERE

This script sets MTU to 576 bytes (decimal) for ppp connections (IPMTU) and pptp connections (TunnelMTU).

See .REG files for background information on .REG files.